  Citation: Hidayat, E.; Harada, H.; Mitoma, Y.; Yonemura, S.; A Halem, H.I. Rapid Removal of Acid Red 88 by Zeolite/Chitosan Hydrogel in Aqueous Solution. Polymers 2022, 14, 893. https://doi.org/10.3390/ polym14050893 Academic Editor: Hai-Feng (Frank) Ji Received: 31 January 2022 Accepted: 22 February 2022 Published: 24 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Rapid Removal of Acid Red 88 by Zeolite/Chitosan Hydrogel in Aqueous Solution Endar Hidayat 1,2 , Hiroyuki Harada 1,2, *, Yoshiharu Mitoma 1,2 , Seiichiro Yonemura 1,2 and Hadi Imran A Halem 1,2 1 Graduate School of Comprehensive Scientific Research, Prefectural University of Hiroshima, Shobara 727 0023, Japan; hidayatendar1@gmail.com (E.H.); mitomay@pu-hiroshima.ac.jp (Y.M.); yone@pu-hiroshima.ac.jp (S.Y.); hadiimran95@gmail.com (H.I.A.H.) 2 Faculty of Bioresources Science, Department of Life and Environmental Science, Prefectural University of Hiroshima, Shobara 727-0023, Japan * Correspondence: ho-harada@pu-hiroshima.ac.jp Abstract: In the present study, we developed a new adsorbent product with zeolite crosslinked chitosan (ZL–CH hydrogel) to remove acid red 88 (AR88) in an aqueous solution. The effects of several factors, such as the comparison of ZL–CH hydrogel and the absence of chitosan, pH, adsorbent dosage, initial AR88 concentration, contact time, and ion strength, were determined. Obtained results showed that ZL–CH hydrogel improved AR88 removal compared to the absence of chitosan, with an adsorption capacity of 332.48 mg/g in equilibrium time of 1 min, and adding ionic strength had no significant effect. However, with optimal conditions at pH 2.0, dry ZL–CH became hydrogel due to protonation of amino and hydroxyl groups through hydrogen bonds in the AR88 solution. Volume fraction and interaction force decreased with increasing porosity, leading to an increase in adsorption capacity and swelling ratio. Experimental data of the adsorption process showed the Freundlich isotherm model. The equilibrium for adsorption and swelling kinetics studies showed and fitted a pseudo-second-order model. NaOH was successful as a desorbing agent with 93.8%, and it followed the pseudo-second-order kinetics model. The recycling process indicates great potential for AR88 removal. Keywords: acid red 88; adsorption; chitosan; dye removal; zeolite 1. Introduction Water contamination is a significant global problem caused by chemical industry effluents such as textiles, printing, food, pharmaceuticals, and paints [1]. Textile industries are one of the main contributors to water pollution, generating 10–15% of toxic dyes [2], which hurt human health [3]. As a result, dye contamination in wastewater is a significant issue, as it prevents light from passing through the water, reducing photosynthesis, and disrupting biota growth, including fish [4]. Among these, azo dyes are widely dominant and comprise 60–70% [5]. Azo dyes are generally identified by the presence of two aromatic groups followed by coupling the resultant diazonium salt with an electron-donating aromatic compound connected to double bond –N=N– [6]. Numerous techniques are studied for the removal of dyes in an aqueous solution, such as ion exchange [7], adsorption [8–12], membrane filtration [13], fungal decolorization [14], and coagulation or flocculation [15]. However, each approach has its own set of disadvantages. Of these, adsorption is a simple method that uses an adsorbent [16,17]. Many adsorbents are successful because of their ability. Recently, scientists have focused on finding an adsorbent that is nontoxic, efficient, and faster even at higher concentrations. Zeolites are crystalline aluminosilicate minerals with a negative surface charge neutral- ized by exchangeable cations [18]. Furthermore, zeolites have a high surface area [19], which was explored to remove azo dyes [6]. However, zeolites are quite effective in removing azo Polymers 2022, 14, 893. https://doi.org/10.3390/polym14050893 https://www.mdpi.com/journal/polymers