1. Introduction
The release of toxic metal cations to the ecosystem through industrial wastewater is an alarming situation. The presence of such metal ions is a threat to animals and humans due to its toxic nature in both soil and water [
1]. These metals ions entered the aquatic environment through wastewater coming from different industries such as the fertilizer industry, mining, ore, refining, electroplating, painting, batteries, and tanneries [
2]. One of the most toxic metal ions is lead, which is considered as a natural toxin. Lead is mostly released from factories, metal recycling, lead–acid batteries, and pipes [
3]. Its exposure can cause reproductive, renal, hematopoietic, and central nervous system disorders in the human body [
4]. Acute exposure mostly results in headaches, nausea, emotional disruption, and cognitive changes [
5]. Cadmium, a heavy metal ion, has an ample noxiousness and harmful impact on most of the organ systems [
6]. Cadmium is considered as one of the toxic substances to humans and can cause death. Exposure to cadmium through soil, water, and air can cause cancer, respiratory, central, and peripheral nervous, urinary, and skeletal disorders [
7]. Copper is a crucial component for human body, acting as a cofactor of a variety of enzymes such as cytochrome oxidase, ceruloplasmin, superoxide dismutase, dopamine β-hydroxylase, and tyrosinase. However, excess copper and other toxic metal ions can disturb the capability of the liver for disrupting excess copper contents inside the human body. Thus, eventually affecting the organ system such as nervous, reproductive, and adrenal functions, connective tissues, the learning ability of newborn babies, etc. [
8,
9,
10,
11].
Various physical and chemical processes have been developed for the decontamination of these toxic metal ions from industrial effluents such as filtration [
12], solvent extraction [
13], precipitation, ion exchanger [
14], reverse osmosis [
15], oxidation/reduction [
16], filtration [
12], electrochemical techniques [
17], and sorption [
18,
19]. Currently, sorption is advocated as one the most efficient, low-cost, and simple techniques for eliminating toxic metal ions from aqueous solution. It is currently perceived as a successful and monetary strategy for the expulsion of contaminants from water and wastewater systems. Therefore, there is a need for synthesizing an efficient, low cost, simple, and environmentally friendly sorbent for cations remediation from wastewater [
20]. For the decontamination of these toxic metal ions from wastewater, a variety of synthetic as well as natural polymers is being manufactured or modified chemically. Sorbents obtained from biopolymers of natural origin have immensely being used to remove heavy metal ions from industrial wastewater and related aqueous environments [
21]. Biopolymers can completely decontaminate metal ions from industrial effluents. The class of said biopolymers subsumes cellulose, chitin, chitosan, lignins, alginate, and carrageens [
22,
23,
24].
After cellulose, chitin is considered the most abundant natural polymer obtained from the arthropod’s exoskeleton, mostly crabs (
Malacostraca) and insects (Insecta) [
25]. Chitosan is a linear polysaccharide and deacetylated derivative of chitin [
24]. It has been intensively applied in food industries, biomedical, biotechnological, pharmaceutical, cosmetics, agricultural, and water treatment [
26]. Chitosan is biocompatible, environmentally friendly, antibacterial, and biodegradable; it also demonstrates high sorption capacity toward metal ions because of the chemical composition of amino and hydroxyl groups [
24,
27]. The degree of deacetylation is the main controlling factor for sorption capacity [
28]; it also affects the properties of chitosan, which are mainly physical, chemical, and biological. The alteration of natural polymers is an effective technique for the manufacturing of new materials to present extraordinary properties and amplify the field of the plausible uses of those large existing biopolymers [
29]. Different strategies have been utilized either to change characteristic chitosan physically or artificially to boost up the sorption limit [
30]. In earlier studies, it has been confirmed that the available functional amino hydroxyl entities at the second and sixth carbon locations empower an assortment of chemical alterations [
31]. Chitosan (CH) has been alluring on the grounds that the available functional entities impart the cationic and chelating characteristics, which are useful for its functionalization and/or to engineer functionalized CH-based constructs. The chemically altered CH has a more prominent take-up limit with respect to substantial metal ions [
32].
The presented investigation was objected to chemically modify chitosan with glycidyl methacrylate [
23], followed by reaction with 1,4-bis(3-aminopropyl) piperazine in a second step to increase the number of Lewis basic centers for decontamination from aqueous solution. The aim of the work is to prepare a green sorbent with a high sorption capacity for cations. The biodegradability and nontoxicity of the chitosan and modified products make them sustainable tools for wastewater treatment. After characterization, the as-prepared material was subjected to investigate its sorption capacity for copper, cadmium, and lead metal ions in aqueous solutions. The chemically modified chitosan’s sorption limit was additionally assessed by contemplating the equilibrium sorption isotherm for copper, cadmium, and lead with a batch-wise procedure. The sorption capacity of the newly engineered material was compared with the raw chitosan. Moreover, to get the equilibrium information, several sorption isotherms, i.e., (1) the Langmuir, (2) the Freundlich, and the (3) Temkin and parameter values of these isotherms were also acquired in this study.
2. Materials and Methods
2.1. Chemicals, Reagents and Materials
Chitosan (78% of deacetylation, Mwt 195 kDa) powder was obtained from the Primex Ingredients A.S. (Norway). Ethanol (Synth), 1,4-bis (3-aminopropyl)piperazine (Sigma-Aldrich, St. Louis, MO, USA), copper, cadmium, and lead nitrates all were of analytical quality and were used without further treatment.
2.2. Procedure for Modification of Chitosan
The present study involves a two-step reaction to modify the CH for cations sorption from aqueous environment. Firstly, CH modification was performed using glycidyl methacrylate by the reaction of its epoxy ring with the amino group of chitosan, which leads to the opening of the three-member ring through nucleophilic substitution reaction. In neutral pH, the amino group of chitosan is most favorable nucleophilic center. The reaction was reported for the first time using environmentally friendly solvent water [
22]. In the second step, this product, which contains a double bond of the glycidyl methacrylate, acts as Michael’s acceptor in a reaction with 1,4-bis (3-aminopropyl) piperazine using triethylamine as a catalyst. In a typical procedure, CH (4.0 g) was taken in 250-mL three-necked flasks containing 200 mL of water. The suspension was continuously shaken for 15 min at 353 K. After getting a homogenous mixture, 2.65 mL of glycidyl methacrylate was slowly incorporated and placed under constant stirring for 2 h. The resultant material (CH-gly) was filtered, repeatedly washed with ethanol, and dried under a vacuumed environment for 6 h at 318 K. In the second phase, 3.0 g of CH-gly was added to 150 mL of ethanol in a three-necked flask of 250 mL under constant stirring at 333 K. Then, 3.0 mL of 1,4-bis (3-aminopropyl) piperazine was added, followed by the addition of 1.0 mL of catalyst, namely triethylamine (Et
3N). For completion of the reaction, the above mixture was mechanically stirred for up to 72 h. After the designated reaction time, the chemically modified chitosan (CH-APN) was filtered and ethanol washed, which was followed by deionized water washing, and then dried under a vacuumed environment for 6 h at 318 K. The schematic illustration of the whole chemical reaction is shown in
Figure 1.
2.3. Characterization Studies
The elemental analysis of pristine and modified CH material was carried out using a Perkin Elmer model PE 2400 elemental analyzer. Fourier-transform infrared spectroscopy (FTIR) was performed using a Bomem Spectrophotometer (MB-series). All test samples, i.e., pristine CH, and the requisite products of modified chitosan, i.e., CH-gly and CH-ANP were prepared as KBr pellets. Then, the test samples containing KBr pellets were used to record the FT-IR spectra at the wavelength range, i.e., 4000–400 cm−1 with 32 scans and 4 cm−1 resolution.
The solid-state 13C NMR spectra of the samples, i.e., pristine CH, CH-gly, and CH-ANP materials was performed using a Bruker AC 300/P spectrometer. The evaluations were recorded at 75.47-MHz frequencies, along with 4-kHz magic angle spinning, 5-s pulse repetitions, and a 1-ms time of contact. Shimadzu XD-3A diffractometer (35 kv, 25 mA) was utilized for X-ray diffraction patterns, in 2θ form between the range of 1.5° and 50° with nickel-filtered CuKα radiation, and 0.154-nm wavelength. A Shimadzu TGA 50 apparatus was used for thermogravimetric curves, argon-filled air at a 1.67 cm3 s−1 flow rate, with a 0.167 K s−1 heat rate. The cation sorption extent was persuaded by figuring out the diversity between the initial concentration in the aqueous solution and that of the supernatant.
2.4. Sorption Experiments
The batch process was used to investigate the metal ions sorption ability of the modified chitosan from aqueous solution. In a typical procedure, 25.0 mL of metal solution was taken in a different polyethylene flask, and 20 mg of the newly prepared material was added to it. The concentration of the metal solution used in the experiment ranged from 7.0 × 10
−4 to 2.0 × 10
−3 mol dm
−3 at neutral pH. In order to obtain the period associated with the saturation of the isotherms, kinetic experiments were previously performed using similar cationic solutions. The suspension was shaken by an orbital apparatus at 298 ± 1 K for 24 h to reach isothermal saturation. This time was previously established by using the same procedure to obtain an isotherm with a well-defined plateau, indicating that all basic centers in each biopolymer were saturated by cations. An isotherm having a well-defined plateau was obtained by a similar method after 4 h of contact time, demonstrating the saturation of basic centers on chitosan. The number of cations sorbed was determined by separating the supernatant solution from the solid material through decantation. The aliquot obtained was analyzed using ICP-OES (Inductively coupled plasma-optical emission spectrometry) for the presence of cations, and the quantity of the cations sorbed in the sorption process was calculated (mmol g
−1) by Equation (1).
where
Nf,
ni, and
ns represent the number of mole of cations at the equilibrium stage, while
m represents the mass of the sorbent [
33]. In addition, during the whole sorption process, the linear and nonlinear methods were applied to study the Langmuir, the Freundlich, and the Temkin isothermal behavior.