3.1 FE-SEM analysis
Morphology of the synthesized PV/S-g-3D-GO/N was determined by SEM (Scanning Electron Microscope) (Fig.1A). The obtained result shows that all the particles have porous spherical shape and this property helped to better adsorption properties. Moreover, the nanoparticle shows accumulated shape. In addition, it is demonstrated that the particle size is ranged from 40 to 80nm and roughly, spherical shape, however, a slight aggregation phenomenon occurred. The TEM images show that the surfaces of GO are densely sheltered with uniformly distributed black colored Fe3O4 nanoparticle. The Fe3O4 nanoparticle are distributed on GO. Moreover, there is not much agglomeration of Fe3O4 nanoparticles on the surface of graphene oxide. The Fe3O4 particle was founded on the edges of the sheets since carboxyl groups associated at the edges of GO (Fig. 1B).
3.2 FTIR analysis
The FTIR spectra of the Fe3O4 nanoparticle was shown in Fig.2A. The peak around 3387 cm-1 and 634 cm-1 revealing the stretching vibration Fe3O4 group bonded with the O–H group at 1626 cm-1 due to the adsorbed water on the surfaces(12). Fig. 2B is related to FTIR spectra of the 3D-GO/N. As it can clearly see the strong peak on the 3412 cm-1 is observed, which is related to 3D-GO/N vibration, Moreover, the vibration band at 1626 and 634 cm-1 attributed to the O–H and Fe3O4 groups and indicate that surface of nanoparticle have been successfully functioning. The FTIR spectrum of the PV/S-g-3D-GO/N confirms broad bands at 3423 cm-1 that are related to O-H or N-H stretching vibration (Fig.2C). In addition, absorption bands of OH on the surface were observed. Another vibration peak at the 3022 cm-1 and 2916 cm-1 assigned to the C-H bending mode of aromatic and aliphatic group in styrene, respectively. In addition, the peak at 1735 cm-1 was related to the C=O vibration and peak at 1596 cm-1 represented the C-C aromatic bonds. The CH2 and C-O peaks appeared at 1446 cm-1 and 1022cm-1, respectively. The sharp peak of 696 cm-1 also represented Fe3O4.
3.3 XRD analysis
According to obtained result from Fig.3 (A), 3D-GO/N (A) and the PV/S-g-3D-GO/N (B) was characterized by energy dispersive analysis of X-ray (EDX). The result of EDX spectrum prevalent that the planes (220), (311), (400), (511) and (440) which was related to Fe3O4 peaks (30). Moreover, the results show that the formation of the shoulder on its crystals will have no effect on the spectrum of magnetic particles(31). According to obtained result from Fig.3 (B), confirms the purity of Fe3O4 nano- magnetic particles, and the comparison between spectra shows the same structure in the two materials. The average crystallite size of the modified nano-adsorbent was calculated to be 12.55nm using the Scherer-Debye equation.
3.4 Magnetic properties measurement
The Vibrating Sample Magnetometer (VSM) was used to determine the magnetic properties of the modified nano-adsorbent. As shown in Fig.4, the 3D-GO/N and PV/S-g-3D-GO/N spectra were S shape. The hysteresis loop of 3D-GO/N and PV/S-g-3D-GO/N is 70emu/g and 32emu/g, respectively. This reduction was due to the coating of N-vinyl caprolactam and styrene on 3D-GO/N, which acted as an adsorption agent and reduced the magnetic properties of the core as a shielding.
3.5 Thermal gravimetric analysis (TGA)
The thermal behavior of 3D-GO/N and PV/S-g-3D-GO/N was studied by thermal gravimetric analysis (TGA) from 0 to 600 °C. As shown in Fig.6 and spectrum a, 3D-GO/N lost less than 1% of its weight due to the evaporation of surface water adsorbed on it at 150 °C. In general, weight loss about 80 to 150°C is due to the evaporation of water adsorbed onto the nanoparticle surfaces(32). Thereafter, no weight loss of up to 400 °C was observed, but at the higher temperatures, the weight loss was due to the burning of graphene oxide. For PV/S-g-3D-GO/N (Fig.5 A&B), the weight loss was less than 1% due to the evaporation of water adsorbed onto the particle surface. The weight was constant up to 300 °C and then decreased due to the removal of the polymeric branches. At 400 °C, weight loss was due to the burning of graphene oxide. The thermal gravimetric analysis showed that the polymeric branches have been successfully applied to 3D-GO/N.
3.6 Effect of various parameters on the adsorption of MPCA
3.6.1 Effect of pH on adsorption
pH is one of the most important parameters in the adsorption process, in this study the effect of different pH (2-8) on MCPA herbicide (10g/L) removal efficiency after 60 min contact time with PV/Sg-3D-GO/N (3g) as an adsorbent was studied. As can be seen in Fig.6, the adsorption process was more efficient at acidic pH because MCPA had an acidic structure. At basic pH, the herbicide was deformed into an ionic form and the adsorption decreased(25).
3.6.2 Effect of adsorbent dose on the adsorption efficiency
The effect of adsorbent dosage on adsorption efficiency was investigated. Obtained results show that adsorption efficiency has increased with increasing dosage of PV/S-g-3D-GO/N (Fig.7). However, unique properties of Graphene based adsorbent make it good candidate for remove different kind of contaminant from aquatic solution and different author reported different dosage for optimum removal efficiency that is different from /L0.1-12 mg for different pollution such as organic and inorganic pollutant (33). The reason is that when the dosage of the PV/S-g-3D-GO/N increased, the adsorption sites and functional groups on the adsorbent surface increased(34, 35) and lead to increasing removal efficiency. This could be due to the increase of active sites and functional groups at the nano-adsorbent surface.
3.6.3 Effect of contact time on the adsorption efficiency
The effect of contact time on the removal efficiency of MCPA at the optimal condition was studied. The results showed that as the contact time increased, the adsorption efficiency increased. Maximum removal efficiency was observed at the 300 min of contact time.
3.6.4 Effect of temperature on the adsorption efficiency
The effect of temperature on the MCPA removal by the PV/S-g-3D-GO/N was investigated by performing equilibrium adsorption studies at five different temperatures in the range of 30-55°C. As shown in Fig.9, removal efficiency, increased with increasing temperature.
3.6.5 Effect of initial MPCA concentration on the adsorption efficiency
Figure.10 shows the adsorption efficiency of the PV/S-g-3D-GO/N for the removal of MPCA in different initial MPCA concentrations (5–60 mg L−1) at the optimal condition. The experiments, the parameters affecting the adsorption were stabilized at optimum values obtained from the previous steps (pH: 3, of adsorbent dose: 5 g/L, temperature 50 °C, and contact time of 300 min). The results show that the removal efficiency decreased with increasing concentration of herbicide in solution.
3.7 Isotherm and kinetic studies of the adsorption
Adsorption isotherms indicate the interaction between adsorbate and adsorbent in the liquid phase (free adsorbate solution) concentrations and the solid phase (adsorbent-attached solute) concentrations at constant temperature (36). Therefore, Langmuir, Freundlich, Temkin and Redlich-Peterson models were used in this study. In addition, the pseudo-first-order, pseudo-second-order was used for the evaluation kinetic of adsorption (Table.1). The obtained data reveal that the Langmuir model (R2 = 0.998) yielded best-fits to the experimental data(Table.2) that is maybe due to mono layer and the surface of the adsorbent surface is uniform. Therefore, there is no interaction between adsorbed molecules(37). The positive values of ΔS also indicated that the degree of freedom at the solid-liquid intermediate was increased during adsorption (Table.3)(38).
3.8 Adsorbent regeneration
Fig.11 illustrates the percentage change in adsorbent capability for the removal of herbicide after regeneration. As can be seen in the figure, the PV/Sg-3D-GO/N could operate up to twice usage with 100% efficiency. Then its efficiency was reduced but still could be used by up to seven times by more than 50% efficiency. Since the process of MCPA removal by adsorbents was based on surface adsorption, it is likely that the adsorbent may be used more frequently by changing the type of solvent used or increasing the washing time.
3.9 Application of PV/S-g-3D-GO/N in the removal of MCPA herbicides from real samples
After the adsorption process under optimum conditions, the residual MCPA concentration in the samples was below its detection limit (1 ng/L). These results showed that the synthesized adsorbent had excellent performance in removing MCPA from real samples and therefore impurities in water and wastewater samples did not significantly interfere with removal of the target contaminant.
3.10 Comparison of PV/S-g-3D-GO/N adsorption capability with 3D-GO/N
In order to determine the effectiveness of surface modifications and its branching on MCPA herbicide adsorption from aqueous solutions, PV/Sg-3D-GO/N coincidence with 3D-GO/N at optimum conditions (pH: 3, adsorbent dose: 5 g/L, temperature: 50 °C, contact time: 300 min) was used to remove herbicide from water. The results showed that the removal percentage of MCPA from water with PV/Sg-3D-GO/N was equal to 83.12±0.1 while this value was in relation to the 3D-GO/N of 25.66 ±7.37. Therefore, if the PV/Sg-3D-GO/N adsorption power of 100% is considered, the 3D-GO/N has 30% ability to remove MCPA from water. So it can be concluded that surface modification and branching of 3D-GO/N were effective in the adsorption of MCPA herbicides, which could be due to the accumulation of fine particles and the greater ability of styrene as herbicidal adsorption agent in the adsorbent surface.