Abstract
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.
Highlights
Synthetic dyes are widely used in various sectors, such as the textile, leather, and paper industry, and it has been estimated that thousands of tons of different dyes are produced yearly [1]
The chemical crosslinking of chitosan represents a crucial step in the preparation process
It implies that part of the functional groups of chitosan that are involved in the crosslinking reaction cannot be available as active sites for adsorption, lowering the overall performances of the resulting material
Summary
Synthetic dyes are widely used in various sectors, such as the textile, leather, and paper industry, and it has been estimated that thousands of tons of different dyes are produced yearly [1]. Due to the large volumes of dyed wastewaters, the recalcitrant nature of most dyes and their high tendency to contaminate water even in small concentrations, water pollution by dyes is considered a serious environmental issue [2,3,4,5]. Among the possible removal strategies, adsorption is claimed as one of the most feasible, versatile, and low-cost methods, relying on the physical and/or chemical interactions between the targeted molecules and the adsorbing substrate [7]. The main characteristics of an ideal adsorbent material are large specific surface area, high porosity, great adsorption capacity, as well as simplicity of production, stability, and low cost and environmental impact [8]
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