Zwitterion Adsorbent of Crosslinked Chitosan-Benzil/Algae/Coal Fly Ash for Anionic (Remazol Brilliant Blue R) and Cationic (Thionine) Dyes Removal: Box–Behnken Design Optimization

In this work, a zwitterion adsorbent which consists of crosslinked chitosan-benzil/algae/coal fly ash (CS-BZ/Alg/FA) was developed for the removal of two structurally different anionic dye (remazol brilliant blue R (RBBR)) and a cationic dye (thionine (THN)). The physicochemical characteristics of CS-BZ/Alg/FA were investigated by several analytical techniques including specific surface area, XRD, FTIR, FESEM and EDX analyses. Moreover, response surface methodology and Box–Behnken design (RSM–BBD) was adopted as a statistical tool to optimize the adsorption operational parameters for THN and RBBR dyes removal. Thus, the optimal adsorption conditions were determined as follows: CS-BZ/Alg/FA dose of 0.1 g/100 mL, solution pH 10 for THN, and solution pH 4 for RBBR. Adsorption equilibrium isotherm data for RBBR and THN dyes by CS-BZ/Alg/FA were well described by Freundlich (multilayer adsorption) and Langmuir (monolayer adsorption) isotherm models respectively. Thus, the maximum adsorption capacities of CS-BZ/Alg/FA towards THN and RBBR were found to be 69.0 mg/g and 259.9 mg/g, respectively at 25 °C. Moreover, the pseudo second order model accurately represents the kinetic data for the adsorption of both THN and RBBR dyes. The adsorption thermodynamic functions indicate the adsorption process of THN and RBBR was spontaneous and endothermic in nature. Ultimately, this research presents the CS-BZ/Alg/FA as an eco-friendly amphoteric adsorbent with preferable capability for capturing anionic and cationic dyes from aqueous environment.