Chitosan/sodium alginate aerogels showing effective adsorption of anionic Congo Red and cationic Methylene Blue dyes for sustainable water treatment

Abstract

Water pollution caused by organic dyes poses a significant environmental concern. To address this issue, chitosan (Ch)–sodium alginate (SA) aerogels were synthesizedusing a straightforward method and evaluated as bio-based adsorbents for model dyes. Three formulations were prepared by varying the Ch-to-SA weightratios: Ch-25-SA-75, Ch-50-SA-50, and Ch-75-SA-25. The aerogels displayed remarkable structural stability, retaining over 80 % of their mass after 24 h of immersionin solutions with pH values ranging from 4 to 11. The adsorption performance was evaluated for anionic Congo Red (CR) and cationic Methylene Blue (MB) dyes. TheCh-50-SA-50 sample exhibited the highest adsorption capacity for CR after 24 h, reaching 81.2 mg/g at an initial dye concentration of 200 mg/L. The Ch-25-SA-75sample exhibited the highest adsorption capacity for MB, reaching 59.6 mg/g at an initial dye concentration of 100 mg/L after 24 h. The modeling and interpretationof the adsorption tests showed that when CR was adsorbed, the kinetic behavior follows the pseudo-first order model and the isotherms, the Freundlich and Sipsmodels, while when MB was adsorbed, the kinetic behaviors follow the pseudo-second order model and the isotherms, the model proposed by Sips. A simpleregeneration process involving ethanol and hydrochloric acid or sodium hydroxide solutions was developed to attempt to extend the lifespan of the adsorbents.