Tailoring Chitosan/LTA Zeolite Hybrid Aerogels for Anionic and Cationic Dye Adsorption

Chitosan (CS) is largely employed in environmental applications as an adsorbent of anionic dyes, due to the presence in its chemical structure of amine groups that, if protonated, act as adsorbing sites for negatively charged molecules. Efficient adsorption of both cationic and anionic dyes is thus not achievable with a pristine chitosan adsorbent, but it requires the combination of two or more components. Here, we show that simultaneous adsorption of cationic and anionic dyes can be obtained by embedding Linde Type A (LTA) zeolite particles in a crosslinked CS-based aerogel. In order to optimize dye removal ability of the hybrid aerogel, we target the crosslinker concentration so that crosslinking is mainly activated during the thermal treatment after the fast freezing of the CS/LTA mixture. The adsorption of isotherms is obtained for different CS/LTA weight ratios and for different types of anionic and cationic dyes. Irrespective of the formulation, the Langmuir model was found to accurately describe the adsorption isotherms. The optimal tradeoff in the adsorption behavior was obtained with the CS/LTA aerogel (1:1 weight ratio), for which the maximum uptake of indigo carmine (anionic dye) and rhodamine 6G (cationic dye) is 103 and 43 mg g^-1, respectively. The behavior observed for the adsorption capacity and energy cannot be rationalized as a pure superposition of the two components, but suggests that reciprocal steric effects, chemical heterogeneity, and molecular interactions between CS and LTA zeolite particles play an important role.