Application of magnetic chitosan composites for the removal of toxic metal and dyes from aqueous solutions

Highlights

• Outline of preparation, characterization and properties of magnetic chitosan adsorbents

• Inorganic and organic pollutant removal from water using MCCs has been reviewed.

• Adsorption kinetics and equilibrium isotherm models were reviewed.

• Magnetic separation and adsorption mechanism have been discussed.

• The outlook for potential applications and limitations was also discussed.

Abstract

Magnetic chitosan composites (MCCs) are a novel material that exhibits good sorption behavior toward various toxic pollutants in aqueous solution. These magnetic composites have a fast adsorption rate and high adsorption efficiency, efficient to remove various pollutants and they are easy to recover and reuse. These features highlight the suitability of MCCs for the treatment of water polluted with metal and organic materials. This review outlines the preparation of MCCs as well as methods to characterize these materials using FTIR, XRD, TGA and other microscopy-based techniques. Additionally, an overview of recent developments and applications of MCCs for metal and organic pollutant removal is discussed in detail. Based on current research and existing materials, some new and futuristic approaches in this fascinating area are also discussed. The main objective of this review is to provide up-to-date information about the most important features of MCCs and to show their advantages as adsorbents in the treatment of polluted aqueous solutions.

Introduction

Currently, water is one of the most vital human resources and has economic, social, political and environmental importance throughout the world [1], [2]. Water pollution has become a serious environmental problem and has attracted global concern in recent years, particularly since various pollutants are entering aquatic systems as a result of rapid industrialization and urbanization [3]. Pollutants that are of primary concern include metals, dyes, biodegradable waste, phosphates and nitrates, heat, sediment, fluoride, hazardous and toxic chemicals, radioactive pollutants, pharmaceuticals and personal care products [4], [5], [6], [7], [8]. Trace amounts of any of these compounds lead to an enormous pollution problem and consequently, the treatment of wastewater is a subject of paramount importance. Researchers in the analytical, environmental and material sciences have attempted to develop processes for removing various pollutants and recently, adsorption has become a widely used technology for the removal of both inorganic and organic material [9], [10], [11]. A number of materials have been investigated extensively for this purpose [12], [13], [14], [15], [16], [17], [18], [19], [20] and activated carbon has undoubtedly been the most popular adsorbent used widely throughout the world for the removal of various pollutants [21], [22], [23], [24]. However, activated carbon is relatively expensive, and this has restricted its application at times. Alternative cost-effective sorbents are required for the treatment of metal-contaminated waste streams.

The use of adsorbents composed of natural polymers has attracted significant interest, and polysaccharides such as chitosan and its derivatives have received particular attention [25], [26], [27], [28], [29], [30], [31]. This is due to the fact that chitosan is a low-cost and effective adsorbent compared with activated carbons and other adsorbents used in treatment organic or inorganic contaminated water [26], [28]. There is an abundance of citations in the literature describing the performance of chitosan as an adsorbent for pollutants from water and wastewater including metals, dyes, phenols, fluoride, and phthalates [26], [27], [32]. Due to the unique polycationic structure of chitosan, it has proved to have outstanding removal capacities for anionic dyes such as acid, reactive and direct dyes [28], and because of its cationic behavior, the protonation of amine groups in acidic media leads to adsorption of metal anions by ion exchange. The chelating properties of chitosan and chitosan derivatives toward metal ions have been studied and discussed by Muzzarelli and colleagues [25], [33], [34]. Chelation can be attributed to the presence of a large number of functional groups (e.g., acetamido, primary amino, and/or hydroxyl groups) that are capable of coordinating with different metal ions. The selectivity of chitosan is a highly desirable property, because alkali metal ions and alkali-earth metal ions (abundant but not hazardous) are not chelated, while transition and post-transition metal ions (typically present at trace levels and highly toxic) are sequestered [35].

There is an increasing number of review articles published on the use of chitosan and its derivatives as adsorbents for the removal of contaminants from water/wastewaters [36], [37], [38], [39]. In 2004, Varma et al. [40] published a review that outlined the various classes of chitosan derivatives and compared their ion-binding abilities under varying conditions, and described analytical methods, the sorption mechanism, and a structural analysis of the metal complexes using a variety of methods. Miretzky and Cirelli [41] reviewed Hg(II) removal from water using chitosan and chitosan derivatives. Wan Ngah et al. [30] reviewed the adsorption of dyes and heavy-metal ions using chitosan composites. Muhd Julkapli et al. [42] presented an in-depth review of the preparation, properties, and applications of chitosan-based biocomposites/blend materials, including all essential methodological aspects and applications. Wu et al. [43] reviewed the experimental verification of chitosan and its derivatives as adsorbents for select heavy metals. Bhatnagar and Sillanpää [26] discussed chitin and chitosan derivatives for the removal of various pollutants from water and wastewater.

After adsorption, it can be difficult to separate chitosan-based adsorbents from the aqueous solution using traditional separation methods such as filtration and sedimentation, because adsorbents may block filters or be lost. Furthermore, the adsorbents were discarded with the process sludge, which generates secondary pollution. To overcome the problems related to the ease of separation and regeneration of adsorbents, recent research has been focused on magnetic separation technology. Separation technologies employing magnetic adsorbents are an alternative method for treating water/wastewater that has received considerable attention in recent years [44], [45], [46], [47], [48]. The main advantage of this technology is that a large amount of wastewater can be purified in a very short period of time using less energy and producing no contaminants [49]. Magnetic separation was first described by William Fullarton in 1792 as a method of separating iron minerals [50]. During the 1970s and 1980s, scientists were beginning to realize that materials with magnetic properties were useful for the separation of metal pollutants that are sensitive to magnetic fields from a variety of matrices. In 1996, Towler et al. [51] reported the recovery of radium, lead, and polonium from seawater samples using a magnetic adsorbent consisting of manganese dioxide coated magnetite. Since then, other applications of magnetic materials have been reported [52], [53], [54].

Magnetism is a unique physical property that can be exploited in water purification by influencing the physical properties of the contaminants in water [55]. In combination with other processes, it facilitates an efficient purification of water/wastewater. Magnetic composites adsorb contaminants from aqueous effluents and can subsequently be separated from the medium through a simple magnetic process. Furthermore, by chemically modifying the polymeric shells of these magnetic composites, the functionality can be tailored for various applications in diverse fields such as high-density data storage, magnetic resonance imaging (MRI), drug delivery, therapy, diagnosis, bioseparation, and enzyme immobilization [56], [57], [58], [59], [60], [61], [62], [63], [64].

A number of reviews on chitosan and its derivatives that deal with the elimination of metals and organics using chitosan-based adsorbents are currently available [36], [37], [38], [39], [43]. Recently, Wan Ngah et al. [30] briefly discussed the application of chitosan/magnetite composites for the treatment of heavy metal ions and dyes. However, to date, only one review article describes the use of magnetic chitosan materials for the removal of toxic pollutants, and this review was published more than three years ago [65]. In order to address the complexity of the many factors that influence the adsorption process besides its development, and in light of the continuously increasing number of scientific publications within this area, the available reviews no longer adequately cover all of the important aspects of magnetic chitosan materials in adsorption processes. Hence, this article attempts to summarize recent studies that address the removal of inorganics and organics using magnetic chitosan adsorbents. This review describes the development of magnetic composites using chitosan as a base material, and summarizes recent developments in the preparation and properties of MCCs. Furthermore, we examine various MCCs that are capable of removing toxic substrates from wastewater and describe their characteristics, advantages, and limitations, as well as discuss the adsorption mechanisms involved. The goal of this review is to direct attention to emerging and novel research involving magnetic chitosan materials that might be relevant to future projects for the treatment of wastewater, as well as their use in the medical and analytical fields.