Facile preparation of ionotropically crosslinked chitosan-alginate nanosorbents by water-in-oil (W/O) microemulsion technique: Optimization and study of arsenic (V) removal

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Abstract

Chitosan-alginate nanosorbents were prepared by cross-linking of chitosan and alginate with tripolyphosphate and calcium chloride, respectively following the water-in-oil (w/o) microemulsion cross-linking technique. The prepared nanosorbents were characterized by FTIR spectral analysis that provided spectral evidences for interactions between the chitosan and alginate biopolymers, and binding of arsenic ions with the nanosorbent. The transmission electron microscopy (TEM) revealed that the size of nanosorbent fell into the range of 20–110 nm whereas the scanning electron microscopy (SEM) indicated almost spherical morphology of the nanosorbents. The size of the nanosorbents was determined by dynamic light scattering measurements which also confirmed the nanosize of the sorbent particles. The nanosorbents were employed for removal of arsenic ions from aqueous solutions and the effect of various experimental conditions such as varying metal ion concentration, pH and temperature of the metal ion solutions, doses of the nanosorbent, and chemical composition of the nanosorbents were studied on the percent removal of arsenic ions. The adsorption data was applied to Langmuir and Freundlich adsorption isotherm models, and the kinetics of the adsorption process was also studied. A plausible mechanism of adsorption of arsenic ions was also proposed.

Introduction

Among various toxic metal ions, the arsenic ranks at the top level due to its severe hazardous effects on human lives [1]. This metal causes serious physiological disorders such as skin cancer, liver, kidney, and bladder cancers, conjunctivitis, melanosis, hyperkeratosis, black foot, and the limbs and malignant neoplasm and this is the reason the arsenic has been one of the most studied metal ions by chemists and environmentalists [2(a),(b)]. The intensity of toxicity of arsenic can be realized from the fact that the maximum contamination limit (MCL) of arsenic in drinking water has been fixed at 0.5 mg/L by the World Health Organization [3]. The major pathway of ingestion of arsenic into human blood is via drinking water which is recognized as a huge sink for this pollutant. There are, however, various sources also that discharge this toxic metal into water bodies and cause environmental pollution. The major sources of arsenic include metallurgical operations in smelting of cobalt, lead, and copper, agricultural active agents like arsenic trioxide, calcium arsenate as herbicides, and lead arsenate as insecticide [[4], [5], [6]]. Arsenic is also known to function as metabolic inhibitor and develops iron deficiency in the body. As far as the toxicity potential of arsenic is concerned, it mainly exists in As (III) and As (V) oxidation states. Whereas the former oxidation state of arsenic occurs as H3AsO3 in the reducing environment, the later form of arsenic exists as H2AsO4 and HAsO42− in the oxidizing environments. It is also known that As (III) is more toxic than As (V) and has greater mobility in the environment [7].

The severe toxic effects of arsenic have motivated researchers worldwide to explore new technical know-how for its economical and efficient removal from drinking water [8]. Some of the traditional techniques employed for arsenic removal include filtration, sedimentation, solvent extraction, coagulation, foam-flotation, electrolysis, and membrane process [[9], [10], [11], [12], [13], [14], [15]]. Most of these removal techniques are less efficient, time consuming, and comparative expensive and, moreover, the removal efficiency greatly varies with pH of the metal ion solution [16]. Thus, it is essential to have an appropriate removal method that should be fast, inexpensive, and efficient over a wide range of pH. One of the simplest options to achieve such a removal technique is the adsorption method which has been extensively used in removal of toxic metal ions due its ease of simplicity, high efficiency, cost effectiveness, and reusability of the adsorbent. For instance, Alqadami et al. designed an adsorbent with unique combination of magnetic nanoparticles and metal-organic framework and carried out removal of U (VI) and Th (IV) ions from aqueous solutions [17]. AL-Othman et al. [18] prepared activated carbon form peanut shell and studied the removal of Cr (VI) ions through the adsorption technique. A new and efficient nanocomposite adsorbents composed of Fe3O4 and 3-[2-(2-Aminoethylamino) ethylamino] propyl-trimethoxysilane (TAS) were prepared by Alqadami et al. [19] following a silanization reaction and removal of Cd (II), Cr (III) and Co (II) ions was studied by carrying out adsorption of these three toxic metal ions onto the newly synthesized adsorbent (Fe3O4@ATS). Liang et al. [20] synthesized composite hydrogel beads of aminopropyltriethoxysilane-modified magnetic attapulgite@chitosan (APTS-Fe3O4/APT@CS) and studied the adsorption of Pb (II). The authors observed that the modification of Fe3O4 /APT improved its adsorption capacity. Fan et al. [21] synthesized core-shell nanospheres of polyethylenimine-functionalized sodium alginate/cellulose nanocrystal/polyvinyl alcohol ((PVA/SA/CNC)@PEI) and employed them for the adsorption of diclofenac sodium. In another study, Zhang et al. [22] prepared magnetic beads of bentonite/carboxymethyl chitosan/sodium alginate and studied the adsorption of Cu (II) ions from aqueous solution.

Although the subject of arsenic removal has been excellently reviewed [23,24], some of the key studies undertaken for the removal of arsenic using different adsorbents may be highlighted as follows: Deng et al. [25] prepared titanium oxide anchored magnetic sheets of iron oxide and investigated the removal of arsenic (III) and (V) ions from aqueous solution. Di lorio et al. [26] synthesized magnetite nanoparticles by partial oxidation of hot iron nitrate solution and studied the removal of arsenic from aqueous solutions. The authors reported that the magnetic nanoparticles prepared at pH 6.56 exhibited much higher removal capacity of arsenic. Verduzco et al. [27] prepared solid graphene composites by electrodeposition technique and studied the removal of chromium and arsenic ions from water. Ma et al. [28] fabricated iron oxide coated single walled carbon nanotubes and obtained high adsorption capacity of arsenic ions at pH 4. Wang et al. [29] prepared magnetic nanoparticles embedded chitosan beads and employed them for the removal of arsenic ions. In an interesting study, the copolymers of vinyl imidazole and methacrylic acid were synthesized by microwave synthesis and arsenic removal was investigated under different experimental conditions [30]. Shim et al. [31] fabricated manganese oxide impregnated alginate beads and used the prepared nanocomposites for removal of cadmium and arsenic ions. The authors reported that the surface area of the prepared adsorbent was twofold higher than that of the native alginate beads.

The survey of literature reveals that in most of the studies the adsorbents were either the oxide materials or oxide-impregnated polymers. The use of nanostructures composed of naturally occurring ionic polymers in removal of arsenic are less studied. In a study by Chen and Chung [32] the adsorption of As (V) and As (III) were performed on chitosan by batch and column methods and the effect of various experimental factors on the removal of arsenic was examined. An adsorption capacity of 1.9 mg/g was reported for As (V) adsorption. The use of alginate, an anionic biopolymer, for removal of arsenic was attempted by Hussain et al. [33] and an adsorption capacity of 39.4 mg/g was obtained at 20 °C. The lower values of adsorption capacities of chitosan and alginate, respectively led the authors to explore the possibility of getting higher adsorption capacity for arsenic if both the biopolymers are integrated to yield nanoparticles. Thus, the major objectives of the present study include preparation of chitosan-alginate nanoparticles (CAN) by water-in-oil (w/o) microemulsion mediated crosslinking of biopolymers, characterization of nanoparticles using analytical techniques like FTIR, TEM and SEM/EDX, determination of porosity, and surface area, and study of the removal of arsenic from aqueous solutions through adsorption under different experimental conditions.

Section snippets

Materials

Sodium alginate and chitosan were purchased from Merck India and used as received. Sodium tri-polyphosphate (Loba Chemie, Mumbai, India) was used to crosslink positively charged chitosan macromolecules while calcium chloride (High Media, India) was used to crosslink alginate polyanion via ionotropic gelation reaction. In order to prepare water-in-oil microemulsion the required oil phase was prepared from liquid paraffin oil (Merck, India). Other chemicals used such as rhodamine B (indicator),

Optimization of CANPs size

Since the nanoparticles were prepared following a water-in-oil microemulsion technique, the knowledge about the experimental factors that influence the size of the droplets of the microemulsion is of great significance as it would be helpful in preparing nanoparticles of desired dimension. In the present study, therefore, the effect of agitation tine and volume ratio of aqueous to oil phases on the size of CANPs was studied as discussed below:

Conclusions

The inter and intra molecular crosslinking of chitosan and alginate in w/o microemulsion by tripolyphosphate (TPP) and calcium chloride produces nanoparticles of chitosan-alginate (CANPs) which can be optimized for their sizes by varying the experimental conditions like speed of agitation, and volumes of aqueous and oil phases. The speed of agitation up to 60 min results in an increase in the size while beyond 60 min the size of nanoparticles decreases. The increase in volume of aqueous and oil

Declaration of Competing Interest

The author declares that he has no conflict of interest of any type.

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