3.1. FTIR Analysis
The FTIR spectra of FeS NP and FeS/chitosan-g-poly(acrylamide) nanocomposite are shown in Fig. 1 (A, B). Figure 1A showed a wide peak at 3332 cm− 1 attributed to O-H vibrations from water molecules absorbed by the sample. Peaks at 1632 cm− 1 and 621 cm− 1 were as a result of S-S vibrational stretch from the FeS nanoparticle [28]. A peak at 1133 cm− 1 was characteristic of the S-O stretching vibration. In the fingerprint region, vibration bands corresponding to Fe-S stretch were observed below 500 cm− 1 [29]. The FTIR of the grafted nanocomposite (Fig. 1B) showed a broad absorption band at 3188 cm− 1 which was due to N-H vibrations of amide groups and O-H stretching of chitosan coinciding. A band at 1543 cm− 1 was ascribed to the (C = O) vibrations of secondary amide [30, 31]. A band at 1401 cm− 1 was due to C-N vibrational stretch which was an indication of the grafting between chitosan and polyacrylamide [30]. Peaks found below 500 cm− 1 were as a result of Fe-S stretching mode [22, 29]. The shift in peak positions of Fe-S bands indicated the interaction of the copolymer with the nanocomposite.
3.2. XRD Analysis
Fig 2 shows the XRD patterns of the as-synthesized FeS NP and FeS/chitosan-g- poly(acrylamide) nanocomposite. The peak positions (2θ values) at 17.56⁰, 28.83⁰ and 40.75⁰ corresponded to reflections from the (001), (101) and (111) planes of the tetragonal FeS phase (JCPDS: 86-0389) [29]. The grafted nanocomposite showed lower intensity in the peaks as compared to the FeS NP. This may be due to the interaction of the polymers (chitosan and polyacrylamide) with the nanoparticle and the less crystalline nature of the nanocomposite, compared to the nanoparticle [31]. From the Sherrer equation, the crystallite size of the FeS nanoparticle was found to be 6.2 nm.
3.3. SEM and TEM analysis
(Fig. 3A, B) shows the SEM and TEM micrographs of the as-synthesized FeS nanoparticle. The as-synthesized FeS nanoparticles were seen to be agglomerated and spherical or globular shaped. FeS NP synthesized through conventional methods, tend to agglomerate rapidly to macroparticles due to the interparticle van der Waals interaction [32]. Similar spherical shaped results were reported by other researchers [28, 33–35]. Figure 3C, D shows SEM image of FeS/chitosan-g-poly(acrylamide). nanocomposite and the SAED pattern of FeS NP obtained from the TEM microscope respectively. The SEM of the graft nanocomposite revealed that FeS NP was coated by the copolymer. The flake-like appearance was due to the grafting of FeS and polymers (chitosan and acrylamide) [31]. Though not all of the chitosan and acrylamide were fully grafted.
The SAED pattern of FeS (Fig. 3D) showed small spots forming rings ascribed to the Bragg reflections from the individual crystallite of the FeS nanoparticle indicating polycrystalline nature of the particles [36].
3.4. Adsorption Removal Experiments
3.4.1. Effect of EY concentration, FeS/chitosan-g-poly(acrylamide) dosage and contact time
From the graph (Fig. 4A), the removal efficiency decreased from 92.32–86.56% with an increase in EY concentration. However, the amount of dye adsorbed per unit mass of the adsorbent increased from 55.39 mg g− 1 to 129.84 mg g− 1 with an increase in EY concentration. The decrease in R.E may be due to the fewer number of available surface-active sites as the concentration increased. The increase in Qe may be ascribed to a huge driving force of the EY particles. At lower concentrations, fewer adsorbate particles are attached to the surface of the FeS/chitosan-g-poly(acrylamide) nanocomposite but as the concentration increased, the mass transfer driving force towards the adsorbent surface increased.
Fig 4B shows the effect of adsorbent dose on the adsorption of EY onto FeS/chitosan-g- poly(acrylamide) nanocomposite by varying the dosage in 50 ml of the dye solution. The removal efficiency increased from 78.16% to 83.00% up to 0.15 g dose after which there was a gradual plateau in a decreasing manner. The initial increase may be ascribed to the availability of adsorption sites and the plateau and eventual decrease may be a result of saturation of the adsorption sites. However, Qe reduced from 78.16 mg g-1 to 11.48 mg g-1 with an increase in FeS/chitosan-g- poly(acrylamide) dose. This may be as a result of the adsorbent surface sites available to EY overlapping. However, any additional increase in FeS/chitosan-g- poly(acrylamide) dosage had little effect on the adsorption.
The overall trend for both R.E and Qe was the same in Fig. 4C. There was a rapid rise in adsorption between the first 10 to 30 minutes after which there was a gradual rise. The rapid rise in the first 30 minutes may be due to the availability of more vacant adsorption sites.
3.4.2. Effect of Temperature and pH
The decrease in Qe and R.E (Fig. 5A) with rising temperature indicate that the adsorption was an exothermic process. This may be due to a decrease in adsorptive forces between the adsorbate and the adsorbent as the temperature increased.
The amount of dye uptake per unit mass and removal efficiency of EY were strongly affected by the pH of the wastewater as shown in Fig. 5B. Qe and R.E increased gradually. This may suggest that there was a higher affinity between the adsorbate and the nanocomposite at higher pH.
3.5 Adsorption Isotherms
To be able to efficiently utilize the adsorption system for practical applications, the Langmuir, Freundlich and the Dubinin-Radushkevich isotherms were employed in this work.
Figure 6 below shows the isotherm equilibrium data points for adsorption of EY onto FeS/chitosan-g- poly(acrylamide) nanocomposite.
From the graphs, the R2 value of the Langmuir isotherm was 0.9591, Freundlich isotherm was 0.9657 and that of Dubinin-Radushkevich was 0.8243. It can be concluded that Freundlich isotherm fitted the system since the R2 value was high.
The mean free energy of adsorption, E, was computed to be 0.408 kJ mol− 1 which was within the range of 0 < E < 8 kJ mol− 1 thus the adsorption was physical.
3.6. Adsorption Kinetics and Thermodynamic studies
To ascertain which kinetic order explains the adsorption of Eosin yellow onto FeS – chitosan-g- acrylamide composite, the contact time studies were evaluated using the pseudo-first order and the pseudo-second-order kinetics. The best fit results were obtained in the pseudo-second-order model with an R2 value (0.9997) closer to unity as compared to the pseudo-first order R2 value of 0.9673. Hence, it was established that the adsorption followed a pseudo-second-order process. A similar trend on the adsorption of Eosin Y was also reported [6, 37].
Table 1 below summarizes the thermodynamic studies of the adsorption process
Table 1
Thermodynamic parameters for EY adsorption onto FeS/chitosan-g-poly(acrylamide) nanocomposite
∆H°(kJ mol− 1) | ∆S°(kJ mol− 1) | ∆G°(kJ mol− 1.K) | | | | |
| | 300 K | 303 K | 308 K | 313 K | 318 K |
-12.243 | -0.023 | -5.492 | -5.252 | -5.152 | -5.005 | -5.078 |
The negative ΔHo value indicated an exothermic process which correlates to the adsorption studies on the impact of temperature on the adsorption process. The negative value of ΔSo showed reduced randomness during the adsorption process. It also suggests that there was no substantial change going taking place in the internal structures of FeS/chitosan-g- poly(acrylamide) composite during the adsorption process [38]. The negative values of ΔGo suggested the spontaneity of the adsorption process.