Decomposition of Bis(N-benzyl-salicydenaminato)zinc (II) Complex for the Synthesis of ZnO Nanoparticles to Fabricate ZnO-Chitosan Nanocomposite for the Removal of Iron (II) Ions from Wastewater

e whole world is faced with a huge challenge of the shortage of clean water due to industrialization and the intimidation of climate change. Poor water quality distresses many areas of human’s well-being. Although there are existing technologies for water treatments, many of these methods utilize toxic substances which create more problems into the environment. e preparation of bis(N-benzyl-salicydenaminato)zinc (II) complex and the synthesis of zinc oxide nanoparticles via the thermal decomposition of zinc complex together with the fabrication of ZnO-chitosan nanocomposites for the removal of iron (II) ions from wastewater is reported. e optical properties of the synthesized nanoparticles showed band edges that are red-shifted in wavelengths when the decomposition temperature was increased. e XRD patterns displayed the hexagonal ZnO phase for the synthesized nanoparticles. TEM images revealed spherical-shaped particles which became agglomerated when the temperature was increased. e parameters such as pH, contact time, and initial concentration were investigated during the water treatment. e pH � 6 was found to be optimum, and the highest percentage removal was recovered after three hours for both adsorbents.


Introduction
e high level of metals in our aqueous systems may cause major problems to the environment, humans, and animal lives [1].e increase of the utilization of the heavy metals in the industry sectors has resulted in the increase of metallic substances in natural water sectors [2].e existence of these metal ions such as iron is perhaps the most common water difficulty that is faced by water authorities.Generally, iron occurs in two oxidation states, reduced divalent ferrous (Fe(II)) and oxidized trivalent ferric (Fe(III)) [3].e removal of toxic heavy metals such as iron from the environment and wastewater systems has become an important challenge in our days.e removal process should be simple, operative, and of low cost.Numerous methods have been used to remove Fe(II) ions from wastewater which include complexation, adsorption, bioremediation, biosorption, ion exchange, chemical precipitation, cementation, coagulation and flocculation, and membrane processes [4].However, adsorption has turned out to be one of the alternative treatment techniques for the removal of heavy metals recently since it is promoting the applications of low-cost adsorbents [5].
ZnO is a conventional wide band-gap semiconductor which is environmental friendly and compatible with living organisms.ZnO is also regarded as nontoxic and harmonious with the human skin by forming a suitable preservative for fabrics and surfaces which are associated with skin [6].ese materials can be used in numerous fields such as UV light-emitting devices [7], photocatalysts [8], hydrothermal process [9], cosmetic industries [10], and pharmaceuticals.Several methods have been reported for the synthesis of zinc oxide nanomaterials which include precipitation [11], sol-gel [12], wet chemical synthetic method [13], etc. Nanomaterials have shown great potential for their applications in wastewater treatment.ZnO materials are capable of operating in water treatment via various nanotechnology routes [14].El Saeed and his colleagues synthesized ZnO through direct precipitation technique using Zn(NO 3 ) 2 •6H 2 O and (NH 4 ) 2 CO 3 at 40 °C.e precursor was calcinated for 2 hr at 550 °C.e TEM images showed the nonagglomerated ZnO nanoparticles with an average particle size of 20 nm [15].Kumar et al. reported the synthesis and the use of low band-gap ZnO nanoparticles for water treatment, recently [16].
e preparation of zinc oxide nanoparticles for water disinfection was reported by Dimapilis and coworkers.ey discovered that ZnO can be very effective in various strains of microorganisms [17].e preparation of zinc oxide from modified chitosan and its morphological studies was reported by irumavalavan et al. [18].ey observed that the surface-modified chitosan was showing enhanced surface properties.ere are many other researchers who have developed numerous methods for synthesizing zinc oxide nanomaterials, but the disadvantage of these techniques is that of the usage of toxic and expensive starting materials to prepare these nanoparticles.Iron as a heavy metal in wastewater can be removed by different methods such as electrocoagulation [19][20][21].
e objective of this study was to stress out the substantial application of low-cost materials for the water treatment process.Recently, salicylaldehyde has been used to prepare several substances due to its low cost to form inexpensive materials for biological processes.e preparation of zinc complex from reaction of salicylaldehyde, amine, and metal salt is reported.e metal complex was used to produce ZnO nanoparticles through the thermal decomposition technique which is a simple and very cheap method that produces nanomaterials which are friendly to the environment.
ese nanoparticles were then incorporated with chitosan which is a natural biopolymer material to form ZnO-chitosan nanocomposites.e nanocomposites were utilized in the treatment of wastewater to remove Fe(II) ions.e percentage removal of iron (II) ions from water was explored using pure chitosan solution and ZnO-chitosan nanocomposites.e effect of pH, contact time, and initial metal ion concentration was investigated.

Experimental
2.1.Materials.Zinc acetate dihydrate, salicylaldehyde, benzamine, methanol, acetone, toluene, chitosan, and ferrous ammonium sulfate hexahydrate.All the chemical substances and reagents used in this study were of analytical grade and were utilized without any further purification.

Preparation of the Zinc Complex [C 26 H 20 N 2 O 2 Zn].
e complex was produced using the reported procedure [22].Briefly, an equimolar of benzamine (10 mmol) was added into salicylaldehyde (10 mmol) in methanol followed by the addition of zinc (II) acetate (5 mmol) in methanol into a 100 mL two-necked flask.e mixture was refluxed at a temperature of 50 °C for 3 hrs.e formed precipitate was then filtered, washed three times with acetone, and dried in a desiccator.e product was obtained as a light yellow solid, m.pt.178 °C, CHNO analysis: Calc.: C, 68.20

Synthesis of Zinc Oxide Nanoparticles.
A sample of the dry zinc precursor (0.5 g) was added into a 5 mL cleaned and dried ceramic boat.e boat was then transferred inside a combustion tube which was then transported into the tube furnace under the fume hood.e sample was calcinated at a temperature of 300 °C under the inert environment for an hour.e product was allowed to cool down to room temperature in the furnace.e method was repeated at 400 and 500 °C to investigate the effect of temperature.e resulting zinc oxide nanoparticles were dispersed in toluene for further analysis.

Preparation of ZnO-Chitosan Nanocomposites.
e synthesized ZnO nanoparticles (∼3 mg) were dispersed in 10 mL of distilled water.
e filtered solution was then transferred into a beaker with 0.5% chitosan (50 mL) solution.
e mixture was enclosed with a foil and ultrasonicated for 3 hr to ensure complete combination.e prepared ZnO-chitosan nanocomposite solutions were then further utilized in water treatment.

Batch Adsorption Experiments.
A solution of heavy metal (1000 ppm) of ferrous ammonium sulfate was prepared and used as a stock solution.Different concentrations of this solution ranging from 1 to 50 ppm were prepared and used for adsorption studies at room temperature to investigate optimum conditions such as the effect of pH, concentration, and contact time in a 100 mL beaker.ZnOchitosan nanocomposites or 0.5% chitosan (5,00 mL) solution was added into the stock solution.e pH of the solution was audited to the anticipated values by adding 0.1 M NaOH or 0.1 M HCl.

Characterization 2.6.1. Fourier Transform Infrared (FTIR) Spectroscopy (FTIR).
e FTIR analysis of zinc complex was conducted at room temperature in a scan range between 400 and 4000 cm −1 from a Bruker FTIR tensor 27 spectrophotometer.

Elemental Analyzer (EA).
Microanalysis was accomplished on a PerkinElmer automated model 2400 series II CHNS/O analyzer.

ermogravimetric Analysis (TGA).
A PerkinElmer Pyris 6 TGA that was conducted at 20 °C min −1 heating rate from 30 to 900 °C in a closed perforated aluminium pan under nitrogen gas was utilized for thermal analysis.

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UV-Visible Spectroscopy (UV-Vis) and Photoluminescence (PL).
e absorption and emission spectra of ZnO nanoparticles were obtained using UV-1800 Shimadzu spectrophotometer and Gilden Fluorescence.

Transmission Electron Microscopy (TEM).
Transmission electron microscopy (TEM) images were collected from a Tecnai F30 FEG TEM instrument at an accelerating voltage of 300 kV.

Results and Discussion
e chemical reaction between salicylaldehyde, amine, and zinc acetate in methanol at a temperature of 50 °C produced bis(N-benzyl-salicydenaminato)zinc (II) complex as shown in Scheme 1. e precursor was then used to synthesize zinc oxide nanoparticles by thermal decomposition.
e incorporation of ZnO nanoparticles into chitosan to form polymer nanocomposites was explored.e nanocomposite and chitosan were then used in water treatment.

Structural Analysis.
Figure 1 displays the FTIR spectrum of bis(N-benzyl-salicydenaminato)zinc(II) complex that shows a strong band at 1590 cm −1 which is due to ν(C�N) of the imine.e strong bands were also detected at 1180-1405 cm −1 which are due to the phenolic ν(C-O) stretching.
e stretching frequencies that confirm the coordination of nitrogen and oxygen atom into metal ion were also detected and allocated at 510 and 450 cm −1 which are attributed to the Zn-O and Zn-N bonds.ese outcomes were similar to the reported results [23].
e thermal decomposition profile of bis(N-benzylsalicydenaminato)zinc (II) complex was investigated using the thermogravimetric analysis linked with differential thermogravimetric analysis (TGA/DTA) (Figure 2).e TGA shows that the prepared zinc complex is stable up to 241 °C and shows a single-stage decomposition arrangement which is shown as a dotted line in the DTA graph. is weight loss of 63% in the single-stage decomposition is in the range of 241-407 °C, which can be attributed to the loss of the ligand moiety with its oxidative degradation to metal oxide, beyond 407 °C. is is due to high degree of electron delocalization through an intricate approach which advances to consistency in bond strength [23].

Optical Analysis.
e optical absorption measurements of the ZnO nanoparticles 300, 400, and 500 °C are shown in Figure 3.
e absorption spectrum of the nanoparticles synthesized at low temperature in Figure 3(a) (i) shows two absorption peaks located at 402 and 423 nm which can be ascribed to the presence of the ligand moiety in the surface of the nanoparticles.e spectrum also shows the band edge at 324 nm, whereas the spectra of the nanoparticles synthesized at higher temperatures (Figure 3(a) (ii)-(iii)) revealed band edges at 356 and 360 nm.All the absorption band edges were blue-shifted in comparison with the bulk ZnO [24].
e band-gap energies were calculated using Tauc's relationship [25].e band-gap can be anticipated by Tauc's relationship between the optical absorption coefficient, α, the photon energy (hv), constant (A), and the direct bandgap energy (E g ) as shown in equation ( 1) below: where n � 0.5 for direct allowed transitions and n � 2 for indirect allowed transition.Plotting (αhv) 2 versus photon energy (hv) and extrapolating the linear portion of the curve to absorption coefficient equal to zero can contribute the band-gap energy (Eg) values.Figure 3(b) displays the bandgap energies that were decreasing with the increase in decomposition temperature.e emission spectra of ZnO nanoparticles (Figure 4) displayed emission maxima at 400, 412, and 429 nm which were red-shifted from the absorption band edges.e emission spectra of the nanoparticles prepared at higher temperatures (400 and 300 °C) show the broad emission peaks which signify the polydispersity of the particles.
Figure 6 shows the TEM images of the synthesized ZnO nanoparticles that are spherical in shape.e synthesized particles were increasing in sizes and become agglomerated when the decomposition temperature was increased due to Ostwald ripening.e average particle sizes were found to be 2.56 ± 0.619, 6.36 ± 1.27, and 8.79 ± 2.43 nm for ZnO nanoparticles prepared at 300 (i), 400 (ii), and (iii) 500 °C, respectively.

Adsorption Studies.
Numerous factors such as pH, contact time, and initial metal ion concentration that affect the removal of Fe(II) ions from wastewater have been investigated.
e solution was filtered after the adsorption processes and analyzed with flame atomic absorption spectrophotometry (AAS).e percentage removal of Fe(II) ions from wastewater was then calculated as reported previously [27] using the following formula equation ( 2  Journal of Chemistry where C o (ppm) is the initial metal ion concentration and C x (ppm) is the final metal ion concentration in the solution.

Effect of pH, Contact Time, and Initial Concentration.
e solutions of chitosan and ZnO-chitosan nanocomposites were used as adsorbents to investigate the effect of pH of 50 ppm of Fe(II) solution (20 mL).e solution was stirred for 10 minutes and filtered.e maximum percentage removal of Fe(II) ion from the solution in Figure 7(a) was discovered to be about 62.99% for pure chitosan solution and 83.37% for ZnO-chitosan nanocomposites when the pH was elevated.e optimal pH was found to be 6.
To study the effect of contact time (Figure 7(b)), 50 ppm of the iron (II) solution (20.00 mL) was poured into different beakers.Each beaker was treated with 5 mL nanocomposites or chitosan and stirred at different times (10, 30, 60, 120, 240, and 480 min).e results showed that 120 minutes was the optimum contact time for both adsorbents which is similar to the reported results by Xaba et al. [28].
Journal of Chemistry concentration was varied from 50 to 200 mg/L.e results show that as the initial concentration is increased, the percentage removal decreases due to the volume gradient influence of the solution and adsorbent which is similar to the reported results [29].

Conclusion
e bis(N-benzyl-salicydenaminato)zinc (II) complex was prepared and characterized using spectroscopic and elemental analysis.e FTIR results showed the expected significant bands, and TGA displayed a single-stage decomposition which confirmed the purity of the complex.e ZnO nanoparticles were successfully synthesized through the thermal decomposition of the zinc precursor.e effect of various calcination temperatures was also studied.e optical properties revealed an increase in wavelength of the band edges and emission peaks when the decomposition temperature was elevated.e results also disclosed that the band-gap energy of the synthesized ZnO can be regulated by ZnO@300°C ZnO@400°C (iii) ZnO@500°C   varying the growth temperature.e XRD patterns exhibited a hexagonal phase for all the synthesized particles.
e TEM images showed that when the temperature was increased, the particles were increasing in sizes.e images also revealed the agglomerated particles at higher temperatures.ZnO-chitosan nanocomposite displayed the better percentage removal of Fe(II) ions compared to chitosan solution.

Figure 7 :
Figure 7: Effect of pH (a), contact time (b), and initial concentration (c) on adsorption of Fe(II) ion using ZnO-chitosan nanocomposites (i) and chitosan (ii) as adsorbents.