Green Synthesis of Iron Oxides and Phosphates via Thermal Treatment of Iron Polyphenols Synthesized by a Camellia sinensis Extract

Abstract

Iron oxide nanoparticles (FeONPs) prepared with plant extracts have been emerging as green and sustainable materials. FeONPs are usually amorphous due to the chelation of the tea polyphenols (TPs) to the iron, and the real nature of the iron compounds is not completely understood. The main goal of this study was to investigate the behavior of the green FeONPs synthesized from an Fe3+ salt and Cammelia sinensis (black tea) extract upon thermal treatment, in order to remove TPs and enable the formation of crystalline materials suitable for a thorough characterization and with the potential for diverse applications. The as-prepared FeONPs were assigned as mixed-valence Fe­(III) oxyhydroxides and Fe­(II)/Fe­(III) ions bound to TPs. A detailed description of the phase transformation upon heating revealed the formation of the rare nano β-Fe2O3 phase at 400 °C, followed by a transformation to α-Fe2O3 as the temperature increased. Above 600 °C, the unprecedented formation of FePO4 and Fe3PO7 was observed, produced from the reaction of Fe2O3 and free phosphate ions present in the black tea leaves, Fe3PO7 being the major phase obtained at 900 °C. Finally, the catalytic potential of the FeONPs to treat the azo dye methyl orange through a heterogeneous Fenton-like system was investigated.

All Shapes and Phases of Nanometer-Sized Iron Oxides Made from Natural Sources and Waste Material via Green Synthesis Approach: A Review

“…147−151 As shown in Figure 5, we even can tune the iron oxides in different phases by simply tuning the temperatures in the same green synthesis process via Camellia sinensis extract. 147 In this case, the phase changes from β-Fe 2 O 3 to α-Fe 2 O 3 occur at a temperature between 500 °C and 800 °C, and α-Fe 2 O 3 remains stable until 900 °C. During the increase in temperature from 600 °C to 800 °C, however, the FePO 4 and Fe 3 PO 7 phases were also formed.…”

“…159 As for the synthesis of Fe 3 O 4 , several biological substance agents were used such as aqueous extracts of zanthoxylum armatum dc leaf, 160 Rhus coriaria, 161 Graptophyllum pictum leaf, 162 Chlorella-k01, 163 seaweed (Kappaphycus alvarezii), 164 Garcinia mangostana fruit peel, 165 gundelia tournefortii l plant., 166 Punica granatum l. of fruit peel, 167 biosurfactant Saccharum of ficinarum, 168 Azadirachta indica, 169 rambutan peel waste, 170 α-D-glucose, and gluconic acid. 171 Green synthesis even could be used to obtain metastable iron oxides, 147,172 for instance, β-Fe 2 O 3 nanoparticles synthesized using a simple chemical method from a mixture of FeCl 3 solution and Iraqi grape extract 172 and β-Fe 2 O 3 nanoparticles synthesized from a Fe 3+ salt and Cammelia sinensis (black tea) extract using heat treatment. 147 Furthermore, iron oxides could exhibit an enhanced functionality when they are in the form of a composite and combined with other materials.…”

“…171 Green synthesis even could be used to obtain metastable iron oxides, 147,172 for instance, β-Fe 2 O 3 nanoparticles synthesized using a simple chemical method from a mixture of FeCl 3 solution and Iraqi grape extract 172 and β-Fe 2 O 3 nanoparticles synthesized from a Fe 3+ salt and Cammelia sinensis (black tea) extract using heat treatment. 147 Furthermore, iron oxides could exhibit an enhanced functionality when they are in the form of a composite and combined with other materials. Green synthesis utilizing biological substance has been reported efficient to obtain iron oxide-based nanocomposites.…”

“…The biological substance also can be used to obtain nanometer-sized iron oxides with specific oxidation states and phases, as well as iron oxide-based nanocomposites. − As shown in Figure , we even can tune the iron oxides in different phases by simply tuning the temperatures in the same green synthesis process via Camellia sinensis extract . In this case, the phase changes from β-Fe 2 O 3 to α-Fe 2 O 3 occur at a temperature between 500 °C and 800 °C, and α-Fe 2 O 3 remains stable until 900 °C.…”

“…In this case, the phase changes from β-Fe 2 O 3 to α-Fe 2 O 3 occur at a temperature between 500 °C and 800 °C, and α-Fe 2 O 3 remains stable until 900 °C. During the increase in temperature from 600 °C to 800 °C, however, the FePO 4 and Fe 3 PO 7 phases were also formed . Indeed, the α-Fe 2 O 3 is known to mostly occur at higher temperatures, compared to other Fe 2 O 3 phases. , The synthesis of specific oxidated iron oxide phases is mostly dependent on the amount of oxygen captured or released during the synthesis process.…”