SYNTHESIS OF Ag-Fe3O4 NANOPARTICLES FOR DEGRADATION OF METHYLENE BLUE IN AQUEOUS MEDIUM

Fe3O4 known as magnetite is one of the oxides of iron which plays a major role in various fields of sciences. Fe3O4 was synthesized by precipitation method using NH3.H2O, FeCl2.4H2O and FeCl3.6H2O as precursor materials. For synthesis of 5% Ag-Fe3O4, the green synthetic method was used for immobilization of Ag nanoparticles on Fe3O4 using leaves extract of Calotropis gigantea plant. The synthesized Fe3O4 and 5% Ag-Fe3O4 were employed as catalyst in degradation of methylene blue. The photo catalytic activity of Fe3O4 was remarkably enhanced by doping of Fe3O4 with Ag nanoparticles. Advanced instrumental techniques including XRD, EDX, TGA and SEM were used for characterization of synthesized particles. The immobilization of Ag on Fe3O4 enhanced the photo degradation of methylene blue from 40 to 72% at 40 °C which confirms that 5% Ag-Fe3O4 is an active catalyst for treatment of dye contaminated water. Ag-Fe3O4 exhibited almost same catalytic activity in two successive cycles.


INTRODUCTION
A large number of elements are known which are plentiful in occurrence and extensively used on earth. Iron, in the form of various oxides, is one of these known elements which has potential for different applications. Various types of oxides of Fe like Fe 3 O 4 , β-Fe 2 O 3 , α-Fe 2 O 3 and FeO are known to exist. These polymorphs of iron oxide exhibit remarkable chemical and physical properties which are favorable in wide range of applications. These magnetic nanoparticles of iron oxides have many uses such as recording material, magnetic resonance imaging, magnetic drug target, environment and catalysts [1]. Fe 3 O 4 known as magnetite is one of the oxides of iron which plays a major role in various areas of chemistry, material sciences, physics and medical sciences. Fe 3 O 4 crystallizes in mixed oxidation state iron (Fe 3+ and Fe 2+ ) inverse cubic spinel structure [2][3][4]. Fe 3 O 4 can be used in magnetic resonance imaging, in drug delivery systems, as sorbent for heavy metal, as antibacterial agents, as catalyst, as electrochemical biosensors, as shielding material in electromagnetic interference and for energy harvesting [5][6][7][8][9]. It has been reported that separation between valence band of Fe(4s) and O(2p) in Fe 3 O 4 is 4-6 eV (309-206 nm) [10]. Hence, Fe 3 O 4 can be confidently tested as photo catalyst for remediation of aqueous contaminants under ultra violet/visible irradiation. The photo catalytic activity of metal oxide, Fe 3 O 4 , can be further boosted up by doping it with metal atoms. In metal-metal oxide photo catalyst, the photo excited electron easily shifts to fermi level of doped metal atom via Schottky contact which prevents the de-excitation of electron and hence it improves the catalytic performance of metal oxide [11][12][13]. In this study we have made an attempt to develop an effective photo catalyst for degradation of methylene blue in aqueous medium.

Synthesis of Fe 3 O 4
Chemical coprecipitation method was used for synthesis of Fe 3 O 4 nanoparticles using NH 4

Degradation procedure
The photo degradation of methylene blue over Fe 3 O 4 and Ag-Fe 3 O 4 was conducted in a Pyrex glass beaker as batch reactor as we reported earlier [13]. For this purpose, a mixture of methylene blue solution and synthesized catalyst was stirred for 15 min in dark. Afterward, the photo reactor was irradiated with visible light in the presence of O 2 . A photo reacted suspension was taken periodically and analyzed with UV-visible spectrophotometer for concentration of methylene blue.

Characterization
The crystalline structure of prepared nano particles of Thermal stability of the prepared Fe 3 O 4 and Ag-Fe 3 O 4 was studied by thermal gravimetric analysis (TGA) (Figure 3). It is concluded that the prepared photo catalyst is stable over a wide range of temperature as no appreciable loss on weight of the sample was noted with temperature over 30-600 °C. About 8% weight loss was observed in TGA analysis of  The shape of the prepared Fe 3 O 4 and Ag-Fe 3 O 4 NPs was investigated by SEM analyses. Considering the micrographs given in Figure 4, it is concluded that prepared particles are dispersed and non-agglomerated. Furthermore, the particles are homogeneous and spherical in shape.

Catalytic activity of synthesized Fe 3 O 4 and Ag-Fe 3 O 4
The catalytic activity of synthesized Fe 3 O 4 and Ag-Fe 3 O 4 was evaluated by visible light assisted photo degradation of methylene blue. The catalytic activity of prepared Fe 3 O 4 and Ag-Fe 3 O 4 on degradation of methylene blue was estimated from measurement of absorbance/optical density of reaction mixture after every 10 min. It was found that absorbance at λ = 630 nm continuously decreased as it is treated with Fe 3 O 4 /Ag-Fe 3 O 4 . The decrease in absorbance shows the degradation of dye. The degradation of dye is expressed in a plot of A t /A 0 (A t = absorbance of reaction mixture treated with photo catalyst for different time, A 0 = absorbance of un-treated reaction mixture) against time as given in Figure 5. It is suggested that photo catalytic degradation of methylene blue dye reported here takes place in two stages. First, the degradation of dye proceeds slowly with generation of the OH radicals as suggested by slight decrease in absorbance with an induction period. The second stage of reaction is dominated with a significant decrease in absorbance. This stage represents degradation reaction where OH radicals mineralize the dye molecules [16]. The degradation of 0.0188 M (50 mL) dye solution

Synthesis of Ag-Fe3O4 nanoparticles for degradation of methylene blue in aqueous medium
Bull. Chem. Soc. Ethiop. 2020, 34(1) 127 obtained after treatment of dye with Fe 3 O 4 and Ag-Fe 3 O 4 for 120 min was 41 and 73%, respectively. To confirm the photo chemical reaction, degradation procedure was repeated in the absence of irradiation under similar conditions. It was observed that treatment of dye solution in the absence of light did not cause any significant change in concentration as given in Figure 6. A small decrease in concentration of dye in the dark is due to adsorption of dye on the surface of catalyst. Therefore, the reaction mixture was stirred for 15 minutes in dark to get the adsorption equilibrium. Moreover, methylene blue was degraded in the presence of Fe 3 O 4 /Ag-Fe 3 O 4 catalyst due to OH radical. Irradiation of photo catalyst leads to formation of OH radical by a series of reactions of positive hole and photo excited electron pair (h + /e -). These OH radical are active and unstable species which react with methylene blue and mineralize it. The proposed mechanism is described by following reactions [13,17,18]:    [19]. Hence the deposition of Ag on Fe 3 O 4 improves the catalytic activity of Fe 3 O 4 . The possibility of proposed mechanism was confirmed by using isopropyl alcohol as OH radical scavenger [20].
The dependency of catalytic activity of Ag-Fe 3 O 4 on temperature was also studied. Typically, separate degradation reactions of methylene blue were conducted with 0.1 g Ag-  The degradation kinetics in present study is described by Eley-Rideal mechanism which suggests that the molecule of methylene blue dye reacts with adsorbed oxygen [21][22][23][24][25][26]. The proposed mechanism can be expressed mathematically in Eq. 2 as given below. Considering the constant pressure of oxygen, Eq. 2 can be written as Eq. 3, which changes to Eq. 4 on integration. The final mathematical expression, Eq. 4, is first order kinetics equation The degradation data of methylene blue at various temperatures was analyzed according to kinetic expression, Eq. 4. The results obtained by this analysis are given in Figure 8. On the basis of best straight lines in this figure (Figure 8), we conclude that photo degradation of methylene blue over Ag-Fe 3 O 4 in this study follow the pseudo-first-order kinetic model. The apparent rate constants determined from the slops of straight lines are 0.0076, 0.0102 and 0.0131 per min at 30, 40 and 50 °C, respectively. Other researchers have also reported similar kinetics analyses [27][28][29][30][31][32][33][34]. 22.2 kJ/mol was calculated as energy of activation using the Arrhenius plot. As photo reactions are not much temperature dependent, therefore the activation energy is low. Similarly, the dependency of catalytic activity of Ag-Fe 3 O 4 on initial concentration of dye was also studied. Typically, separate degradation reactions of methylene blue were conducted at 40 °C with 0.1 g Ag-Fe 3 O 4 using a 50 mL solution of methylene blue having concentration 0.0063, 0.0125 and 0.0188 M. The analyses of reaction mixtures after 120 min showed that about 93, 85 and 73% of the dye degraded with 0.0063, 0.0125 and 0.0188 M, respectively. The data obtained is given in Figure 9. The degradation data with various initial concentrations of methylene blue was analyzed according to Eq. 4 and the obtained results are given in Figure 10. The best straight lines suggest the pseudo first order kinetics in this study as stated earlier. The apparent rate constants determined from the slops of straight lines are 0.0207, 0.0138 and 0.0102 per min with 0.0063, 0.0125 and 0.0188 M, respectively. High concentration of dye imparts an intense color to solution which suppress the infiltration of photon to the solution resulting a decrease in rate of reaction. Hence increase in concentration of dye decreases the rate of reaction [35,36].

ACKNOWLEDGMENTS
Thanks to Government College University Faisalabad Pakistan and The World Academy of Sciences, Italy, for assistance under GCUF-RSP and COMSTECH-TWAS, respectively.