Effect of Camellia sinensis Origin and Heat Treatment in the Iron Oxides Nanomaterials Composition and Fenton Degradation of Methyl Orange

Sustainable and environmentally friendly methods for nanomaterials synthesis have been emerging recently. The use of extracts of polyphenol-rich plants with high reducing and chelating power is advantageous because the polyphenol can protect the nanomaterial from agglomeration and deactivation. Green nanomaterials have been applied in several areas, including remediation of toxic organic pollutants from contaminated effluents. Herein, we describe the preparation of green iron oxide nanoparticles (IONPs) with extracts of the plant Camellia sinensis as black tea for dye removal application. The as-prepared IONPs were composed of amorphous FeOOH and FeII/III-polyphenol complexes. To obtain crystalline and pure iron-based nanomaterials, the amorphous precursor was annealed at 900 ºC. Samples of black tea from different regions were used to verify the reproducibility of the iron phases formed. The same iron phases were obtained for all black tea samples, α-Fe2O3 (hematite), FePO4, and Fe3PO7, but in different proportions. The materials were applied as heterogeneous-Fenton catalysts for the removal of the dye methyl orange. The amorphous as-prepared IONPs were more active than the respective annealed IONPs due to the proton release from the polyphenol into the reaction medium, setting the pH to around 3, which is the optimum pH for the Fenton system.


Introduction
Environmentally friendly synthesis of nanomaterials has been emerging in recent years.2][3] Polyphenols is a class of compounds that includes caffeine, flavanol, theaflavins, thearubigins, catechins and polymeric pigments, among others. 4,5Polyphenols can chelate to the metal ion through the hydroxyl groups and at the same time can reduce, functionalize, and stabilize the nanoparticles formed, which prevents nanoparticles agglomeration, deactivation, and produces more stable nanomaterials. 6][8][9][10][11][12][13][14][15] As reported in the literature, [6][7][8][9][10] most of the green IONPs are amorphous materials due to the high content of polyphenols, which precludes formation of defined crystalline nanomaterials as well as formation of iron oxides of scientific and technological interest.Thermal treatment has been described for green IONPs to favor the formation of crystalline and well-defined iron oxides.For instance, IONPs prepared from Fe III and Aegle marmelos or avocado fruit rind extracts were thermally treated at 400 °C and the formation of α-Fe 2 O 3 were observed. 16,17The reaction of Fe II with Sageretia thea extract led to the formation of highly crystalline and pure γ-Fe 2 O 3 phase after annealing at 500 °C. 18Recently, we have reported the effect of thermal treatment in the range of 400 to 900 °C on the IONPs properties prepared from black tea extract. 19A thorough characterization of the samples allowed the assignment of the as-prepared amorphous IONPs as FeOOH and Fe II /Fe III -polyphenols complexes.After the continuous removal of polyphenols while rising the temperature, the concomitant formation of crystalline iron phases took place.The Fe III oxides α-Fe 2 O 3 and β-Fe 2 O 3 were gradually formed along with the formation of FePO 4 and Fe 3 PO 7 .The phosphorous origin was attributed to the black tea leaves. 19However, a question of whether these iron phases would always be formed from different samples of black tea still needed to be addressed.
The main application reported for green IONPs is in environmental remediation, such as organic contaminant removal from industrial effluents.Improper disposal of toxic and recalcitrant pollutants is a major problem since it brings serious risks to the health of living beings and causes environmental deterioration. 20,210,11 IONPs prepared by a green route with extracts of black tea, 7 Yerba Mate, 6,8 and green tea, 11 were applied in the degradation of the dye methyl orange.Furthermore, the green IONPs exhibit special advantages for the Fenton process, as restoration of Fe II from the reduction of Fe III by the polyphenol; improvement of Fe II stability and prevention of iron sludge formation by the polyphenols 'protection; and enhancement of OH• radicals formation by the hydrogen releasing from the polyphenols that can adjust the reaction pH to around 3, which is the ideal pH for Fenton process operation. 1n this work we have prepared green IONPs with three black tea extracts from different regions and have annealed the samples at 900 °C to provide crystalline iron compounds of interest for environmental remediation.The materials were characterized thoroughly and the effect of the tea origin in the reproducibility of the IONPs properties and phase composition was evaluated.The green as-prepared and annealed iron nanomaterials were applied in the heterogeneous Fenton degradation of the azo dye methyl orange.The degradation kinetics was also studied.

Materials
All chemicals are of reagent grade and were used as received, unless otherwise specified.Black tea samples were purchased from Brazil, United States, and Nepal brands, in local markets in the respective country, and were named as BT1, BT2 and BT3, respectively.The black tea samples were composed of dried Camellia sinensis L. Kuntze leaves and stalks.Iron(III) chloride hexahydrate, Folin-Ciocalteu phenol reagent and hydrogen peroxide (30% water) were purchased from Sigma-Aldrich (St. Louis, MO, United States).Hydrogen peroxide was titrated by the iodometric method before use (9.23 mol L −1 ).Methyl orange (MO) (Casa da Química, Diadema, SP, Brazil) was recrystallized before handling.

Synthesis of the iron oxide nanomaterials
The black tea extracts were prepared by heating 28.8 g of the plant and 480.0 mL of water (60.0 g L −1 ) at 80 °C, under stirring for 60 min.The suspension was filtered under reduced pressure and the extract was used immediately.
The iron nanomaterials were prepared by the addition of 480.0 mL of the black tea extract to 240.0 mL of 0.096 mol L −1 FeCl 3 •6H 2 O (6.22 g) aqueous solution, under stirring, at room temperature, during 40 min.After, the reaction was stirred for 1 h, and the black solid were filtered off, washed with water and ethanol, and dried at 75 °C for 3 days.The materials were named BTn Fe according to the black tea used, which was further calcined at 900 °C for 4 h under atmospheric air, producing the samples named BTn Fe 900 .The procedure was repeated for different black tea brands: Brazil ( BT1 Fe), USA ( BT2 Fe) and Nepal ( BT3 Fe).

Methyl orange degradation tests
The methyl orange degradation tests were carried out in water, at room temperature, under magnetic stirring.In a vial, 10.0 mg of the iron nanomaterial, 2.0 mL of 4.7 × 10 −4 mol L −1 dye solution resulting in a final concentration of C f = 4.7 × 10 −5 mol L −1 , 1.5 mL of 9.23 mol L −1 H 2 O 2 resulting in a final concentration of C f = 0.69 mol L −1 , and water to complete 20.0 mL of total volume were mixed.After a given time, aliquots of 1.0 mL were withdrawal, filtered through a 0.22 µm membrane filter, and the solution analyzed spectrophotometrically using a diode-array UV-VIS Agilent 8453 spectrophotometer (Santa Clara, CA, United States).The decolorization percentage was calculated using equation 1: (1)   where Abs 0 is the absorbance of MO at time = 0, and Abs t is the absorbance of MO at given time (min).

Characterization methods
X-ray fluorescence spectrometry (XRF) was used to determine the elemental composition of the black tea leaves, in a Bruker S8 Tiger (Billerica, MA, United States) instrument under He.The Folin-Ciocalteu method was used to determine the total polyphenol concentration (TPC) of the black tea extracts, and the results were expressed in milligram of gallic acid equivalents per gram of BT leaves (mg GA g −1 ) (Table S1, Supplementary Information (SI) section).
The iron nanomaterials were characterized by X-ray diffraction (XRD) using a Rigaku Miniflex II X-Ray diffractometer (Rigaku ® , Japan), monochromatic Cu Kα radiation (λ = 1.540Å) in the 2θ angle range between 5 and 80°, at a step width of 0.05°, counting 1 min between each step.Fourier transform infrared spectroscopy (FTIR) spectra were acquired using a PerkinElmer Frontier Single & Dual Ranger (Waltham, MA, United States) spectrophotometer, in KBr pellets. 57Fe Mössbauer spectroscopy (WissEl, Starnberg, Germany) were performed at room temperature, in transmission geometry with the samples and the 57 Co:Rh source moving sinusoidally.The hyperfine parameter isomer shift (δ) values were expressed in relation to α-iron foil.Scanning electron microscopy (SEM) images were obtained in a Jeol 7100FT (Japan) (LABNANO/CBBP) microscope equipped with an X-ray energy-dispersive spectrometer (EDS) 80 mm 2 single-shot detector (SDD), Oxford Instruments (Abingdon, United Kingdom), with the samples deposited over conducting FTO (fluorinated tin oxide) glass plates by drop casting a 10 mg mL −1 suspension in isopropyl alcohol over the FTO plate.Thermogravimetric analyses and differential thermal analysis (TGA/DTA) were acquired in a Netzsch (Selb, Germany) thermogravimetric system TG 209F1 Iris.10.0 mg of the sample was placed in an alumina crucible and was heated from 35 to 900 °C, at 10 °C min −1 , under synthetic air (20 mL min −1 ).
The methyl orange degradation was followed by electronic spectroscopy in ultraviolet-visible region (UV-Vis) region, in a diode-array Agilent 8453 spectrophotometer (Santa Clara, CA, United States) in water.Total organic carbon (TOC) analyses of the supernatant after reaction were conducted in a Shimadzu equipment, model TOC-L.Iron leaching of the iron nanomaterial caused by hydrogen peroxide in water at the same concentration of the Fenton tests, was evaluated by flame atomic absorption spectrometry (F AAS) in a iCE 3000 Series model, Thermo Analytica (Waltham, MA, United States) spectrometer.

Iron nanomaterials characterization
The green IONPs were prepared from FeCl 3 and the extract of the polyphenolic-rich plant Camellia sinensis in the form of black tea.We also attempted to investigate the effect of the thermal treatment on the formation of welldefined iron oxides nanomaterials.The reproducibility of the materials before and after annealing in respect to the black tea origin was investigated for three samples from different countries.TPC for the extracts of BT1, BT2 and BT3 was 46.0, 53.6, and 53.1 mg GA g −1 , respectively (Table S1), which is in the range for other plants used in IONPs synthesis. 29Elemental composition determined by XRF showed C and H as the major elements with CH 2 around 98 wt.%, followed by K around 1 wt.%, and Ca, Mg, P and S in the order of 0.1 wt.%.
[8][9][10] SEM images (Figure S1, SI section) showed particles of undefined and irregular shape, with a wide size range due to agglomeration.The corresponding EDS spectra of the sample BT1 Fe in Figure 1b showed carbon and oxygen as the main elements around 60 and 35 at%, respectively.Comparatively, low amount of iron and phosphorous was detected, around 0.2 and 2.0 at%, respectively.Similar elemental composition and distribution was observed for BT2 Fe and BT3 Fe (Figures S2-S4, SI section), as summarized in Table 1.The EDS mapping images showed that the elements are homogenously distributed in all regions of the sample.
Thermogravimetric curves of BTn Fe were acquired under synthetic air and are characterized by three main regions of weight loss (Figure 2).Region I from 30 to 150 °C represents the humidity release around 13 wt.%.The regions II (150-500 °C) and III (500-900 °C) are assigned for degradation of organic compounds of different molecular weight from the polyphenols (Table S2, SI section), accounting for around 78 wt.% of the sample and confirming the high content of organic compounds (Table 1).The tea extracts with higher TPC also provided the higher organic contents (BT2 and BT3).These data are in accordance with the EDS analysis.DTA curves show that the main weight loss takes place around 400-450 °C.Furthermore, around 90 wt.% of the samples is decomposed up to 900 °C, remaining only the IONPs, which accounted for 5.8-7.0 wt.% of iron (Table S2, SI section).
The as-prepared BTn Fe were annealed at 900 ºC, resulting in the BTn Fe 900 samples.The XRD patterns of BTn Fe 900 (Figure 3d) show peaks corresponding to α-Fe 2 O 3 (hematite, JCPDS 33-0664), FePO 4 (rodolicoite, JCPDS 29-715), and Fe 3 PO 7 (grattarolaite, JCPDS 37-61).However, the relative peak intensity of each phase varied according to the black tea used, the major phase in BT1 Fe 900 corresponded to Fe 3 PO 7 , in BT2 Fe 900 corresponded to α-Fe 2 O 3 , and BT3 Fe 900 sample contained the largest amount of FePO 4 .This assignment was corroborated by the refinement of BT3 Fe 900 diffractogram by the Rietveld method (Figure S9, SI section).Moreover, these results agree with the Mössbauer data discussed below.XRD results of IONPs synthesized using Aegle marmelos and avocado fruit rind extracts, also showed formation of the iron oxide α-Fe 2 O 3 when calcinated at 400 °C. 16,17Similarly, the diffractogram of IONPs from Sageretia thea extract showed the formation of pure and crystalline γ-Fe 2 O 3 after annealing at 500 °C. 18he presence of phosphates in the samples can be attributed to the phosphorous element present in the black tea leaves, which upon thermal treatment formed the Fe III phosphates.From XRF analysis (Table S1), phosphorous   accounted for 0.09-0.11wt.% of the tea leaves, that is converted to the FePO 4 and Fe 3 PO 7 phases.Considering the amount of Fe III salt and black tea extract used in the synthesis, and that the BT1 dry leaves have 0.10 wt.% of P in the composition, it would form 0.140 g of FePO 4 or 0.289 g of Fe 3 PO 7 .The amount of BT1 Fe 900 formed after annealing at 900 °C was 0.250 g, close to the value of FePO 4 and Fe 3 PO 7 estimated from the reagents used in the synthesis if all Fe III produced these two phosphates.Fe III phosphates of similar crystalline structures and Mössbauer parameters are commonly found naturally. 30Iron biochar prepared from pyrolysis of dried distillers' grain at 900 °C under N 2 atmosphere also presented phosphites and the phosphorous origin was attributed to the biomass. 31rystallite size of the iron nanomaterials was calculated by the Scherrer equation. 32Crystallites smaller than 36 nm were observed for all iron phases present at the three BTn Fe 900 samples.Comparing the different black tea extracts, it is possible to observe that the nanomaterials presented similar crystallite sizes after the thermal treatment (Table 2).
Figures 3a-3c show the SEM images of BTn Fe 900 , where it is possible to observe that the particles became more defined after the removal of the polyphenols at 900 °C.

BT2
Fe 900 and BT3 Fe 900 adopted a similar spherical morphology, although BT1 Fe 900 still presented more agglomerated, undefined morphology.Particles of 512 nm of diameter could be observed for BT2 Fe 900 , but most of them are outside the nanometer range, probably due to sintering at high temperature.EDS analyses (Table 1) confirmed the drastic removal of polyphenol with residual C around 7 at%, and a considerable increase in the amount of Fe to around 40 at%, P to around 9 at% and O to around 40 at%.The values are typical of the α-Fe 2 O 3 , FePO 4 and Fe 3 PO 7 phases detected by XRD.From the EDS mapping of BT1 Fe 900 (Figure 3e) it is shown that the elements are homogeneously distributed in all regions of the sample.Similar elemental composition and distribution was observed for BT2 Fe 900 and BT3 Fe 900 (Figures S6-S8, SI section).
57 Fe Mössbauer spectra of the as-prepared samples were collected at room temperature (Figure 4) and were properly fitted with three paramagnetic doublets and the corresponding hyperfine parameters of all the subspectra are shown in Table 3.Two of these subspectra, with the hyperfine parameters isomer shift δ ca.0.44 mm s −1 and quadrupole splitting ΔE Q ca.0.81 mm s −1 , corresponds to Fe III (Table 3) in akageneite-FeOOH, or ferrihydrite-FeO(OH)⋅nH 2 O. 6 Also, these two doublets can be attributed  to the presence of ferrolaueite (Fe II Fe III 2 (PO 4 ) 2 (OH) 2 •8H 2 O) 33 or phosphoferrite ((Fe II ,Mn II ) 3 (PO 4 ) 2 •3H 2 O), 33 probably in an amorphous form.The presence of nanoparticles containing phosphorous, as indicated by EDS, support this last attribution for these two doublets.The presence of these minerals justifies the appearance of FePO 4 and Fe 3 PO 7 phosphates after the catalysts are subjected to high temperature thermal treatments (see below).The small doublet with δ ca.1.35 mm s −1 , ΔE Q ca.2.48 mm s −1 and absorption area of A ca. 14% that appears in all as-prepared catalysts, is attributed to the Fe II ions that was chelated and reduced by the polyphenols. 6The formation of Fe II from Fe III promoted by the green tea is explained by the Fe III /Fe II reduction potential (E 0 = +0.77V) compared to polyphenols (E 0 around +0.4 V). 34,35 BT2 presented the highest TPC, followed by BT3 and BT1, what is in line with the Fe II present in the IONPs.Fe II /Fe III -polyphenol nanomaterials prepared from green tea showed a similar Mössbauer spectrum, with two doublets attributed to Fe II and Fe III corresponding to 67 and 33%, respectively. 34 57 e Mössbauer spectra of the samples changed considerably after annealed at 900 ºC (Figure 5).The spectra were fitted with two paramagnetic doublets and a magnetic sextet (Table 4).The doublet with δ ca.0.28 mm s −1 and ΔE Q ca.0.66 mm s −1 corresponds to FePO 4 . 36The second doublet with δ ca.0.33 mm s −1 and ΔE Q ca.1.14 mm s −1 is attributed to Fe 3 PO 7 . 36,37The magnetic sextet parameters are attributed to α-Fe 2 O 3 -hematite. 38Therefore, the Mössbauer experiments are in complete agreement with the XRD results, disclosing the formation of FePO 4 , Fe 3 PO 7 , and α-Fe 2 O 3 after annealed at 900 ºC.The predominant phases in BT1 Fe 900 , BT2 Fe 900 , and BT3 Fe 900 are Fe 3 PO 7 , hematite, and FePO 4 , respectively.The highest amount of FePO 4 phosphate is found in the BT3 Fe 900 sample.From Table 4 we can see that after the annealing, the BT3 Fe 900 sample presents the smallest linewidth and consequently the highest degree of crystallinity. 397][8] The hydroxyl groups of the tea polyphenols (TP) as well as from adsorbed water can be identified in the large band around 3415 cm −1 .The C−H stretching can be observed at 2925 and 2851 cm −1 .The band at 1696 cm −1 is assigned to carboxylic acids, and the following bands at 1628, 1516, and 1453 cm −1 are attributed to the aromatic and aliphatic C−H angular bending modes.C−O bonds show absorptions at 1238 and 1086 cm −1 .The FTIR spectra of corresponding heated samples (Figure 6b) Isomer shift (δ), quadrupole splitting (∆E Q ), linewidth (Γ) and absorption area (A) for the BTn Fe samples (Figure 4).

Degradation of methyl orange by Fenton system
The as-prepared BTn Fe and the annealed samples BTn Fe 900 were applied as catalyst in the heterogeneous Fenton degradation of the dye methyl orange.The tests followed the experimental conditions previously optimized for similar green iron nanomaterials, 7 10.0 mg of catalysts (0.5 g L -1 ), 4.7 × 10 -5 mol L -1 of MO and 0.69 mol L -1 of H 2 O 2 at a final volume of 20.0 mL were used, leading to a MO:H 2 O 2 molar ratio of 1:14500.
Figure 7a shows the UV-Vis electronic spectra of methyl orange degradation catalyzed by BT2 Fe.The decrease of the band at λ max = 464 nm indicates the break of the azo N=N bond.A red-shift of the azo band was observed from 464 to 474 nm after 30 min, and to 490 nm at the end of the reaction.The MO molecule is a pH indicator, so the displacement of the wavelength indicates pH change during the reaction.Accordingly, the reaction pH was monitored, and it lowered from 6.9 before reaction, to 5.9 after catalyst addition, and respectively to 4.4 after H 2 O 2 addition (Table S3, SI section).Finally, after 20 min the pH was 3.4 and continued until the end of the reaction.To confirm that, UV-Vis spectra of aqueous MO solution were taken in different pHs (Figure S12, SI section) and the band wavelength matched with the values observed during the Fenton reaction.These changes are caused by protonation/deprotonation of MO (Scheme S2, SI section), whose speciation depends on the K a = 4.0 × 10 −4 .Below pH 1.0 all molecules of MO are protonated and above pH 5.7 they are completely deprotonated (Table S4, SI section).
According to previous publications, 1,2,[6][7][8]13,14 the polyphenols can release protons during the Fenton degradation, and adjusts the reaction pH around 3, which is the ideal pH for Fenton catalysis. This as corroborated by the increase of TOC during the reaction caused by the partial release of the polyphenols from BT2 Fe (Table S5, SI section), adjusting the pH to 3.4.4. Isomer shift (δ), quadrupole splitting (∆E Q ), linewidth (Γ) and absorption area (A) for the BTn Fe samples (Figure 5).
Figure 7b shows the comparison among all as-prepared catalysts, where more than 90% of degradation was achieved after 5 h.
Control tests were carried out to verify the effectiveness and involvement of the iron nanomaterials in the MO Fenton degradation.A test with the as-prepared and annealed samples and methyl orange without H 2 O 2 presented negligible adsorption capacity of the nanomaterials with less than 4% of adsorption.Moreover, the reaction with MO and H 2 O 2 without the iron catalysts achieved only 14% of degradation, indicating a low contribution of the homogeneous oxidative process, and the importance of the nanomaterial to catalyze the Fenton process.
Metal leaching of an heterogenous catalyst is always a concerning and in the case of Fenton system, H 2 O 2 can accelerate the process.The iron leaching was determined by F AAS, in the supernatant of an aqueous suspension of BT1 Fe or BT1 Fe 900 in presence of H 2 O 2 at the same concentration of the MO degradation tests.BT1 Fe suffered 53% of iron loss during the reaction, representing homogeneous iron concentration of 20.25 mg L -1 , while for BT1 Fe 900 no iron leaching was detected above the limit of detection of the instrument.This estimation is based on the amount of residue from the TG that we assigned to Fe 2 O 3 , and represented 7.0 wt.% of iron for BT1 Fe (Table S2).Iron leaching was also observed for iron oxide nanomaterials prepared with Yerba mate and black tea extracts anchored in amino-functionalized SiO 2 , representing the main drawback of these green nanomaterials. 6o account for the homogeneous contribution from the iron leaching of the catalyst during the Fenton degradation, a control test using FeCl 3 •6H 2 O was carried out.Iron(III) at 50.0 mg L −1 was used and corresponded to twice the concentration of the leached iron.Negligible removal of MO of 10% was achieved, confirming the contribution of heterogeneous catalysis.Furthermore, recent report has demonstrated the improvement of the tea polyphenols in the conventional homogenous Fenton system using Fe II or Fe III ions. 2 The higher lincomycin degradation was attributed to the pH adjustment, but also because the polyphenols can restore Fe II by reduction of Fe III in the reaction medium, which shows the synergy of the system with greater effect than would be expected from the individual contributions of polyphenols, Fe III and H 2 O 2 . 2  The time trace of the azo band decay for the most efficient catalysts BTn Fe were fitted with first and second order kinetic models.Figure 8 shows the decrease of the dye over time for BT2 Fe, which the best fit was obtained for pseudo-first order exponential decay shown in equation 2, leading to higher R 2 values.Figure S13 (SI section) shows the linear plots for first and second order that confirmed a better fitting for the first.The UV-Vis spectra and kinetic plots for BT1 Fe and BT3 Fe are presented in Figures S13  and S15 (SI section).The kinetic data and degradation extension are summarized in Table 5. BT3 Fe was the fastest catalyst, with k obs = 0.0122 min −1 , and led to the highest degradation, 95%.The observed first-order kinetic constant for the BTn Fe catalysts are in the range of published Fentonlike system in degradation of lincomycin (LCM) Fe 0 /H 2 O 2 (k obs = 0.0022 min −1 ), 1 as well as of green FeONPs prepared from tea polyphenols added to Fe 0 for LCM degradation Fe 0 /polyphenol/H 2 O 2 (k obs = 0.0556 min −1 ), 1 and green tea IONPs in degradation of cationic and anionic dyes (k obs = 0.0190 min −1 ). 11) where C 0 is the concentration of MO at time = 0, C t is the concentration of MO at given time (t), and k obs is the firstorder rate constant.
All annealed catalysts BTn Fe 900 were considerably less active than the as-prepared materials, reaching less than 30% of degradation (Figure 9).Furthermore, the azo band was not shifted, and the pH dropped slightly from 6.9 to 6.0 and 4.8, after 20 min and at the end of the reaction, respectively, because of the lack of polyphenols in BT2 Fe 900 . 41hese results reenforces the importance of the polyphenol proton release to adjust the reaction pH to around 3, and the superior Fenton activity shown by the as-prepared catalysts.
Comparing the annealed materials, BT1 Fe 900 was the most active with 26%, and presented Fe 3 PO 7 as the major phase.
In the other hand, BT2 Fe 900 and BT3 Fe 900 presented α-Fe 2 O 3 as the major phase and the lowest activities, 20 and 17%, respectively.Comparing with the IONPs prepared with black tea from Brazil, similar degradation of 34% was achieved.Fenton degradation of methyl orange was carried out using a ultrasmall and conventional α-FeOOH nanorods, achieving a degradation efficiency of 98 and 38% after 60 min, respectively. 42The material BT2 Fe 900 presented the smallest NPs observed by SEM images, and similar efficiency for the ultrasmall α-FeOOH that shows the positive effect on the MO degradation. 42Furthermore, BT2 Fe 900 also presented the highest content of α-Fe 2 O 3 among the IONPs prepared with black tea, probably indicating the effect of this phase in the catalysis.Kinetic data for the annealed catalysts are shown in Figures S16-S18 (SI section).

Figure 1 .
Figure 1.(a) XRD patterns of the as-prepared BTn Fe samples.(b) EDS mapping images of BT1 Fe.

Figure 2 .
Figure 2. TG (a) and DTG (b) curves of the as-prepared BTn Fe samples.

Figure 3 .
Figure 3. SEM images of (a) BT1 Fe 900 , (b) BT2 Fe 900 and (c) BT3 Fe 900 (more images are shown in Figure S5, SI section).(d) XRD patterns of the BTn Fe 900 catalysts treated at 900 °C.(e) EDS mapping of images of BT1 Fe 900 (images of the other materials are shown in Figures S6-S8, SI section).

Figure 4 .
Figure 4. 57 Fe Mössbauer spectra measured at room temperature of the samples BTn Fe.Doublets D2 (blue) and D3 (red) are related to the Fe III ion while the D1 (green) to the Fe II ion (see text).

Figure 5 .
Figure 5. 57 Fe Mössbauer spectra measured at room temperature of the BTn Fe 900 samples.D1 and D2 indicate doublets and S a sextet.The Mössbauer hyperfine parameters of each subspectra are shown in Table4.

Figure 6 .
Figure 6.FTIR spectra of the (a) as-prepared BTn Fe and (b) treated BTn Fe 900 samples at 900 °C (normalized curves are shown in Figures S10-S11, SI section).

Table 1 .
Average atomic percentage a of all catalysts and organic content b of the as-prepared catalysts a Measured from EDS spectra (Figures S2-S8, SI section); b estimated from thermogravimetric data (TableS2, SI section).

Table 2 .
Crystallite size of the iron nanomaterials of the samples BTn Fe 900

Table 3 .
Room temperature 57 Fe Mössbauer hyperfine parameters a

Table 4 .
Room temperature 57 Fe Mössbauer hyperfine parameters a

Table 5 .
Kinetic data for pseudo-first order equation determined by the non-linear fitting, and degradation of the methyl orange obs : first-order rate constant; R 2 : correlation coefficient.