Synthesis of Ag2CrO4/Ag/Fe3O4/RGO nanocomposite as a suitable photocatalyst for degradation of methylene blue in aqueous media: RSM modeling, kinetic and energy consumption studies

Abstract

In this research, an Ag2CrO4/Ag/Fe3O4/RGO nanocomposite (AAFR-NCPs) with unique properties was synthesized. The structural and morphological properties of the AAFR-NCPs were characterized by the FT-IR, XRD, FE-SEM, EDS, DLS, BET, and VSM techniques. The XRD analysis showed an average crystallite size of 7 nm. The FE-SEM images showed that the synthesized AAFR-NCPs possessed a spherical shape and average particle size of 48.5 nm. The DLS results and pHzpc measurement analysis indicated that the surface charge and zero-point charge pH of AAFR-NCPs were at −29.8 mV, and 7.35, respectively. According to the BET results, the average surface area and pore diameter size distribution of the synthesized composite were 73.64 m2/g and 5.86 nm respectively. The corresponding total pore volume of the nanocomposite was 0.2574 cm3/g, which is in good agreement with the estimated results from the BJH method (0.2682 cm3/g). In addition, the VSM results showed that AAFR-NCPs had significant magnetization properties, equal to 41.6 emu/g, which is comparable with the magnetization level of pure Fe3O4 (50.05 emu/g). Moreover, the band gap energy (Eg) of the prepared AAFR-NCPs is estimated to be 1.62 eV. The photocatalytic activity and optimization of the methylene blue (MB) photodegradation process using the synthesized AAFR-NCPs were investigated by the Box Behnken design and by Design Expert software. In this regard, a limited number of influential variables were studied, including the concentration of MB, amount of AAFR-NCPs as the photocatalysts (Cat), pH, and irradiation time (t). A polynomial equation was obtained corresponding to the degradation efficiency (DE) for the experimental data. The calculated R2, Adj-R2, and Pred-R2 values in the model were 0.991, 0.981, and 0.952, respectively indicating a reasonable proportion between the model fitting and experimental data. DE of 97.6 was achieved under optimum conditions of [MB]o = 12 mg/L, [Cat] = 10 mg/L, pH = 2 and ∼103 min time of treatments. The kinetic model under optimum conditions, obeying the pseudo-first-order kinetic model, was studied, with a rate constant (k) of 0.0213 1/min. As the concentrations of MB increased from 5 to 30, the kinetic rate constant decreased 5 times lower as 0.0363 to 0.0073 1/min. Under the obtained optimum conditions, DE irradiation was about 52 % in visible light, which showed the synthesized AAFR-NCPs have good performance for MB degradation in visible light. Finally, under optimal conditions of the process, the electrical energy consumption (EEC) was evaluated and achieved at 450.7 KWh/m3. In addition the criteria for a consistent assessment of the process were proposed and its value of 2.15 × 10−2 was calculated and the results were compared with the other relevant reported processes.