In this thesis, two novel studies are presented, the first one investigated how the Ferrous Reducing Antioxidant Potential (FRAP) of aqueous plant extracts affects F e2+ vacancies (defects) formation within greenly synthesized w ?ustite (FexO) nanoparticles. In the second study, after the complete oxidation of FexO NPs into F e3O4 NPs under ambient conditions, the influence of mediating plant extract's acidity on magnetite surface reactivity has been studied in the preferential and enhanced adsorption of methyl green (MG) on the different greenly synthesized Fe3O4 surfaces by coupling three processes: MG adsorption in ambient dark conditions at the first process, at the second process, the thermocatalysis of MG/Fe3O4 residual solution, and finally the photocatalysis by UV irradiation of MG/F e3O4 residual solution after applying thermocatalysis. This study used four aqueous plant extracts as reducing agents with F eCl3 as a precursor to synthesize FexO NPs. The plants dealt with in this study are Rosemarinus Officinalis.(L), Juniperus Phoenicia.(L), Matricaria Pubescens.(L), and Artemisia Herba-alba.(L), and their acidic extracts pH are respectively; 5.25, 5.05, 4.63, and 3.69. All synthesized samples are characterized by XRD, SEM, ATR-FTIR, and UV-Vis. For FexO, lattice parameters are calculated through XRD data, and then w ?ustite compositions are calculated using the McCammon equation. Results of the first study showed that the FRAP has a clear effect on Fe2+ vacancies formation within w ?ustite NPs. Indeed, the increase in the FRAP leads to a decrease in Fe2+ defects within w ?ustite NPs and consequently an increase in the grain size. Furthermore, studying iron vacancies' effect on w ?ustite formation thermodynamic showed that free energy of w ?ustite formation ?G0f FeO(x) is impacted by cation vacancies and hence by FRAP. The novelty of the second study lies in highlighting the influence of the mediating plant extract on magnetite physicochemical characteristics which impact the preferential and enhanced MG adsorption. Studied characteristics are: hydroxyl groups functionality on magnetite surfaces, grain size, and band gap energy. It was found that plant extract acidity has a clear effect on studied physicochemical characteristics of magnetite samples. The analysis of FTIR spectra showed that hydroxyl groups densities differ on the four magnetite samples. Furthermore, the calculated grain sizes of magnetite samples based on XRD spectra data vary from 29.27 to 41.49 nm.
The analysis of UV-Vis spectra of the four magnetite samples showed that estimated direct band gap energies vary from 2.87 to 2.97 eV. Found results showed that the decrease of mediating plant extract's acidity leads to an increase in functional hydroxyl groups density on magnetite surfaces which conducted to an increase in MG adsorption capacity and yield in the first process. Thus, MG adsorption was more preferred on magnetite surfaces mediated by low acidic plant extracts. Furthermore, the increase of plant extract acidity leads to a decrease in particle size and an increase in band gap energy and therefore to the slowness of electron/hole pairs recombination lifetime upon electrons excitation. So, magnetite greenly synthesized from more acidic mediating plant extract showed more thermo- and photocatalytic activities in MG adsorption. MG adsorption under thermo- and photocatalysis processes were more enhanced on magnetite NPs exhibiting slower electron/hole pairs recombination lifetime.