Magnetic cross-linked chitosan for efficient removing anionic and cationic dyes from aqueous solution

Highlights

• A novel magnetic cross-linked functionalized chitosan adsorbent was synthesized.

• Langmuir, Freundrich and Tempkin models fitted the data at three temperatures.

• Maximum adsorption capacity for CR and CV were 471.5 and 515.9 mg/g, respectively.

• At high concentration, the adsorption process were close to equilibrium at 30 min.

• The adsorbent showed good selectivity, salt resistance and reusability.

Abstract

Herein, a novel magnetic cross-linked chitosan CS-BA@Fe₃O₄ was rationally synthesized by cross-linked with epichlorohydrin and coated with Fe₃O₄ to the acylated chitosan, which was prepared by the reaction of chitosan with benzenetricarboxylic anhydride. The as-obtained absorbent was characterized by FTIR, XRD, VSM, TGA, TEM, BET, SEM and EDS. The results showed that the maximum adsorption capacities of CR and CV were 471.46 ± 16.97 mg/g and 515.91 ± 25.12 mg/g at 318.15 K, respectively. The main adsorption mechanisms were H-bonding and electrostatic interaction. The kinetic data were in good agreement with the pseudo-second-order model and closed to adsorption equilibrium at 30 min. Thermodynamic studies showed that the adsorption on CS-BA@Fe₃O₄ were spontaneous and endothermic. More importantly, the adsorbent exhibited excellent regeneration properties after 6 cycles and remarkable stability under harsh environments including strong acid, strong alkali, multi-salt and mixed dyes conditions. Therefore, abundant efforts revealed a broad application prospect of CS-BA@Fe₃O₄ in water remediation.

Introduction

The deterioration of water by both organic and inorganic contaminants still remains as a challenging issue, which has encouraged extensive investigation in this field of research [1], [2]. Dyes, as a problematic group, are widely used in many industries, such as textiles, paper, printing, leather, paints, plastics, cosmetics and pharmaceuticals [3]. According to reports, there are more than 10⁵ kinds of dyes and pigments on the market, and about 7.0 × 10⁵ to 10 × 10⁶ tons of dyes are produced globally each year, of which about 3.0 × 10⁵ kinds of toxic dyes are released into the aqueous medium through multiple channels every year worldwide [4], [5]. Consisting of benzidine, naphthalene and other aromatic compounds, dyes also exhibit potential hazards such as toxicity and carcinogenicity, even enter into microbial metabolism which threatens the lives of naturals organism over time. [6], [7]. Dyes can generally be divided into the following types: anionic, cationic and non-ionic disperse dyes. Among them, the anionic dye Congo Red (CR) belongs to the aromatic azo type, which can cause health problems such as skin irritation, cancer and human mutations [8], [9]. In addition, a study sponsored by the American Dyestuff Manufacturers Association showed that cationic dye such as Crystal Violet (CV) was generally more toxic than other types of dyes [10]. Generally speaking, dye pollution can lead to serious consequences, so it is urgent to find an effective method to remove the dye in wastewater before it mixed with unpolluted natural water.

In recent decades, some traditional treatment methods, such as coagulation and flocculation, photocatalytic degradation, ion exchange, membrane filtration, electrochemical destruction, reverse osmosis, biological treatment and adsorption, have been developed and widely used in water purification. However, these methods are often controversial due to the poor selectivity, rarely meet the discharge standards, and even produce a large amount of secondary waste [11]. In contrast, the adsorption technology because of its low cost, convenient design and operation, no secondary pollution, the most selective and effective treatment of wastewater, has become the most prominent separation technology to deal with organic dyes and other pollutants in wastewater [12], [13]. Currently, several types of adsorbent materials have been applied to remove dye molecules from water/wastewater including activated carbon [14], clay minerals [15], metal-organic frameworks [16], conducting polymers [17], magnetic nanocomposite [18] and biopolymers, etc. [19].

However, the development of green technology is crucial to removing toxic dyes from wastewater. Therefore, the synthesis of some biopolymer-based materials as effective adsorbents has attracted many researchers. Among various biopolymers, Chitosan (CS) is a deacetylated derivative of chitin, which has wide sources and low cost. Because of its particularity, it has long served to remove toxic substances, heavy metals, and dye molecules [20], [21], [22]. However, when CS is used alone, it has the following disadvantages: (i) unsatisfactory mechanical properties and poor heat resistance, (ii) dissolution in acidic media, (iii) high swelling ratio, and (iv) limited adsorption capacity of basic dyes [23]. In order to overcome the first three limitations, the cross-linked properties can improve the stability of chitosan and mechanical adsorption strength in acidic media [24], [25], [26], [27], [28], and for the last one, build more active sites by chemical modification is necessary to overcome it, the introduction of carboxylate (–COOH), not only increase the solubility, but also can improve the adsorption properties of adsorption capacity by increasing the active sites. However, due to the high dispersion and solubility of small particle sizes, secondary contamination caused by solid-liquid separation difficulties further hinders their ability to achieve green recycling. In addition, traditional separation methods, including centrifugation or filtration, are time-consuming and costly. The adsorbents with magnetic separation properties have created great commercial value because of the convenience of the absorption-fast recovery operation [29].

In this study, chitosan was first acylated with triphenylitic anhydride by introducing carboxyl group to form acylated chitosan (CS-BA), then CS-BA was cross-linked with epichlorohydrin (ECH), and the final adsorbent (CS-BA@Fe₃O₄) was obtained by introducing the prepared Fe₃O₄. The as-synthesized absorbents were characterized by FTIR, XRD, VSM, TGA, SEM and EDS, and investigated systematically the adsorption behavior for removing anionic dye (Congo red (CR)) and cationic dye (Crystal violet (CV)). The Langmuir, Friedrich and Tempkin isotherms were used to evaluate the adsorption equilibrium at different temperatures. The pseudo-first-order, pseudo-second-order equation and intra-particle diffusion model at different concentrations were also studied. The aim was to testify that CS-BA@Fe₃O₄ has potential application prospects in water purification through numerous efforts.