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

Abstract

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 Fe^II/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.

Keywords:
iron oxide; iron phosphate; polyphenols; black tea; methyl orange; Fenton

Introduction

Environmentally friendly synthesis of nanomaterials has been emerging in recent years. The use of extracts of polyphenol-rich plants and biomass residues has gained much attention in the preparation of metal and metal oxides nanoparticles.¹-³ Polyphenols is a class of compounds that includes caffeine, flavanol, theaflavins, thearubigins, catechins and polymeric pigments, among others.⁴,⁵ Polyphenols 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

Green iron oxide nanoparticles (IONPs) prepared with plant extracts have been extensively investigated in recent years.¹-³ Different plants as black tea, Yerba Mate, grape seed, green tea, oolong tea, eucalyptus leaf, have produced different iron nanomaterials because of the different polyphenol composition and synthesis condition, which consequently affects the IONPs properties.⁶-¹⁵ As reported in the literature,⁶-¹⁰ 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-Fe₂O₃ were observed.¹⁶,¹⁷ The reaction of Fe^II with Sageretia thea extract led to the formation of highly crystalline and pure γ-Fe₂O₃ phase after annealing at 500 °C.¹⁸ Recently, 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.¹⁹ A 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-Fe₂O₃ and β-Fe₂O₃ were gradually formed along with the formation of FePO₄ and Fe₃PO₇. The phosphorous origin was attributed to the black tea leaves.¹⁹ However, 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.²⁰,²¹ The removal of dyes commonly found in industrial effluents by IONPs has been extensively studied,²²-²⁸ among them the heterogeneous Fenton system plays a central role to mineralize the contaminant.²,⁶-⁸,¹⁰,¹¹ 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.¹

In 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.

Experimental

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 FeCl₃·6H₂O (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 ^BTnFe according to the black tea used, which was further calcined at 900 °C for 4 h under atmospheric air, producing the samples named ^BTnFe₉₀₀. The procedure was repeated for different black tea brands: Brazil (^BT1Fe), USA (^BT2Fe) and Nepal (^BT3Fe).

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 ${\text{C}}_{\text{f}}=4.7\times {10}^{-5}\text{mol}{\text{L}}^{-1}$, 1.5 mL of 9.23 mol L^-1 H₂O₂ 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) $\text{Decolorization}\left(\%\right)=\frac{{\text{Abs}}_0-{\text{Abs}}_{\text{t}}}{{\text{Abs}}_0}\times 100$

where Abs₀ is the absorbance of MO at time = 0, and Abst 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 = 1.540 Å) 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 ⁵⁷Co: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 mm² 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.

Results and Discussion

Iron nanomaterials characterization

The green IONPs were prepared from FeCl₃ 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 well-defined 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.²⁹ Elemental composition determined by XRF showed C and H as the major elements with CH₂ around 98 wt.%, followed by K around 1 wt.%, and Ca, Mg, P and S in the order of 0.1 wt.%.

The powder XRD patterns of the as-prepared ^BTn Fe materials showed a large peak around 23° (Figure 1a), typical of amorphous IONPs coated with the polyphenols.⁶-¹⁰ 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 ^BT1Fe 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 ^BT2Fe and ^BT3Fe (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.

Figure 1
(a) XRD patterns of the as-prepared ^BTnFe samples. (b) EDS mapping images of ^BT1Fe.

Table 1
Average atomic percentage^a of all catalysts and organic content^b of the as-prepared catalysts

Catalyst	C / at%	O / at%	Fe / at%	P / at%	Organic content / wt.%
BT1Fe	62.5	37.2	0.1	0.2	77.0
BT1Fe900	8.3	56.9	26.9	8.3	-
BT2Fe	46.8	44.9	0.2	0.05	78.3
BT2Fe900	4.4	27.6	57.9	10.2	-
BT3Fe	74.3	23.2	0.2	2.2	80.3
BT3Fe900	7.0	49.1	36.5	6.6	-

• ^aMeasured from EDS spectra (Figures S2-S8, ^SI section); ^bestimated from thermogravimetric data (Table S2, ^SI section).

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).

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

The as-prepared ^BTnFe were annealed at 900 ºC, resulting in the ^BTnFe₉₀₀ samples. The XRD patterns of ^BTnFe₉₀₀ (Figure 3d) show peaks corresponding to α-Fe₂O₃ (hematite, JCPDS 33-0664), FePO₄ (rodolicoite, JCPDS 29-715), and Fe₃PO₇ (grattarolaite, JCPDS 37-61). However, the relative peak intensity of each phase varied according to the black tea used, the major phase in ^BT1Fe₉₀₀ corresponded to Fe₃PO₇, in ^BT2Fe₉₀₀ corresponded to α-Fe₂O₃, and ^BT3Fe₉₀₀ sample contained the largest amount of FePO₄. This assignment was corroborated by the refinement of ^BT3Fe₉₀₀ 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-Fe₂O₃ when calcinated at 400 °C.¹⁶,¹⁷ Similarly, the diffractogram of IONPs from Sageretia thea extract showed the formation of pure and crystalline γ-Fe₂O₃ after annealing at 500 °C.¹⁸

Figure 3
SEM images of (a) ^BT1Fe₉₀₀, (b) ^BT2Fe₉₀₀ and (c) ^BT3Fe₉₀₀ (more images are shown in Figure S5, ^SI section). (d) XRD patterns of the ^BTnFe₉₀₀ catalysts treated at 900 °C. (e) EDS mapping of images of ^BT1Fe₉₀₀ (images of the other materials are shown in Figures S6-S8, ^SI section).

The 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 0.09-0.11 wt.% of the tea leaves, that is converted to the FePO₄ and Fe₃PO₇ 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 FePO₄ or 0.289 g of Fe₃PO₇. The amount of ^BT1Fe₉₀₀ formed after annealing at 900 °C was 0.250 g, close to the value of FePO₄ and Fe₃PO₇ 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.³⁰ Iron biochar prepared from pyrolysis of dried distillers’ grain at 900 °C under N₂ atmosphere also presented phosphites and the phosphorous origin was attributed to the biomass.³¹

Crystallite size of the iron nanomaterials was calculated by the Scherrer equation.³² Crystallites smaller than 36 nm were observed for all iron phases present at the three ^BTnFe₉₀₀ 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).

Table 2
Crystallite size of the iron nanomaterials of the samples ^BTnFe₉₀₀

Catalyst	Crystallite size / nm
	Fe3PO7a	FePO4b	α-Fe2O3c
BT1Fe900	25.9	28.2	35.5
BT2Fe900	26.2	27.2	33.3
BT3Fe900	29.5	29.9	31.5

• ^a2θ = 28.8°; ^b2θ = 25.8°; ^c2θ = 35.5°.

Figures 3a,3b,3c show the SEM images of ^BTnFe₉₀₀, where it is possible to observe that the particles became more defined after the removal of the polyphenols at 900 °C. ^BT2Fe₉₀₀ and ^BT3Fe₉₀₀ adopted a similar spherical morphology, although ^BT1Fe₉₀₀ still presented more agglomerated, undefined morphology. Particles of 512 nm of diameter could be observed for ^BT2Fe₉₀₀, 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-Fe₂O₃, FePO₄ and Fe₃PO₇ phases detected by XRD. From the EDS mapping of ^BT1Fe₉₀₀ (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 ^BT2Fe₉₀₀ and ^BT3Fe₉₀₀ (Figures S6-S8, ^SI section).

⁵⁷Fe 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)·nH₂O.⁶ Also, these two doublets can be attributed to the presence of ferrolaueite (Fe^IIFe^III₂(PO₄)₂(OH)₂·8H₂O)³³ or phosphoferrite ((Fe^II,Mn^II)₃(PO₄)₂·3H₂O),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 FePO₄ and Fe₃PO₇ 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.⁶ The formation of Fe^II from Fe^III promoted by the green tea is explained by the Fe^III/Fe^II reduction potential (E⁰ = +0.77 V) compared to polyphenols (E⁰ 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

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

Table 3
Room temperature ⁵⁷Fe Mössbauer hyperfine parameters^a

Sample	Site	δ / (mm s-1)	∆EQ / (mm s-1)	Γ / (mm s-1)	RA / %	Assignment
BT1Fe	D1	1.95	2.88	0.46	11	FeII
	D2	0.45	0.90	0.56	68	FeIII
	D3	0.43	0.60	0.30	20	FeIII
BT2Fe	D1	1.11	2.64	0.35	15	FeII
	D2	0.48	1.04	0.45	29	FeIII
	D3	0.43	0.65	0.46	55	FeIII
BT3Fe	D1	1.22	2.30	0.95	13	FeII
	D2	0.43	1.05	0.42	39	FeIII
	D3	0.41	0.63	0.44	48	FeIII

• ^aIsomer shift (δ), quadrupole splitting (ΔE_Q), linewidth (Γ) and absorption area (A) for the nFe samples (Figure 4).

⁵⁷Fe 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 5 ca. 0.28 mm s^-1 and ΔE_Q ca. 0.66 mm s^-1 corresponds to FePO₄.³⁶ The second doublet with δ ca. 0.33 mm s^-1 and ΔE_Q ca. 1.14 mm s^-1 is attributed to Fe₃PO₇.³⁶,³⁷ The magnetic sextet parameters are attributed to α-Fe₂O₃-hematite.³⁸ Therefore, the Mössbauer experiments are in complete agreement with the XRD results, disclosing the formation of FePO₄, Fe₃PO₇, and α-Fe₂O₃ after annealed at 900 ºC. The predominant phases in ^BT1Fe₉₀₀, ^BT2Fe₉₀₀, and ^BT3Fe₉₀₀ are Fe₃PO₇, hematite, and FePO₄, respectively. The highest amount of FePO₄ phosphate is found in the ^BT3Fe₉₀₀ sample. From Table 4 we can see that after the annealing, the ^BT3Fe₉₀₀ sample presents the smallest linewidth and consequently the highest degree of crystallinity.³⁹

Figure 5
⁵⁷Fe Mössbauer spectra measured at room temperature of the ^BTnFe₉₀₀ samples. D1 and D2 indicate doublets and S a sextet. The Mössbauer hyperfine parameters of each subspectra are shown in Table 4.

Table 4
Room temperature ⁵⁷Fe Mössbauer hyperfine parameters^a

Sample	Site	δ / (mm s-1)	ΔEQ / (mm s-1)	Bhf / (mm s-1)	Γ / (mm s-1)	RA / %	Assignment
BT1Fe900	D1	0.28	0.69		0.27	13	FePO4
	D2	0.33	1.16		0.28	74	Fe3PO7
	S	0.37	–0.26	51.5	0.72	13	hematite
BT2Fe900	D1	0.29	0.68		0.40	15	FePO4
	D2	0.32	1.16		0.28	28	Fe3PO7
	S	0.37	-0.19	51.9	0.31	57	hematite
BT3Fe900	D1	0.28	0.65		0.29	48	FePO4
	D2	0.38	1.08		0.28	6	Fe3PO7
	S	0.37	-0.20	51.7	0.24	46	hematite

• ^aIsomer shift (δ), quadrupole splitting (ΔE_Q), linewidth (Γ) and absorption area (A) for the nFe samples (Figure 5).

FTIR spectra of the as-prepared ^BTnFe samples (Figure 6a) displayed the main characteristic bands of the polyphenols present in black tea.⁶-⁸ 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) lack the bands of polyphenols and show the characteristic bands of the phases detected by XRD and Mössbauer. The bands between 1700 and 400 cm^-1 correspond to Fe₃PO₇ and FePO₄. The absorptions centered at 1630 and 1462 cm^-1 may be assigned to P=O, and the bands at 1061 and 1029 cm^-1 are due the P–O stretching. The bands at 631, 595, 563, and 435 cm^-1 are assigned respectively to P–O–P, Fe–O stretching and bending deformations (Scheme S1, ^SI section).⁴⁰

Figure 6
FTIR spectra of the (a) as-prepared ^BTnFe and (b) treated ^BTnFe₉₀₀ samples at 900 °C (normalized curves are shown in Figures S10-S11, SI section).

Degradation of methyl orange by Fenton system

The as-prepared ^BTnFe and the annealed samples ^BTnFe₉₀₀ 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 H₂O₂ at a final volume of 20.0 mL were used, leading to a MO:H₂O₂ molar ratio of 1:14500.

Figure 7a shows the UV-Vis electronic spectra of methyl orange degradation catalyzed by ^BT2Fe. 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 H₂O₂ 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 $K_{\text{a}}=4.0\times {10}^{-4}$. Below pH 1.0 all molecules of MO are protonated and above pH 5.7 they are completely deprotonated (Table S4, ^SI section).

Figure 7
Degradation of methyl orange (4.7 × 10^-5 mol L^-1) at 25 ºC catalyzed by 10.0 mg of catalysts: (a) electronic UV-Vis spectra over time for ^BT2Fe, and (b) degradation percentage for all catalysts.

According to previous publications,¹,²,⁶-⁸,¹³,¹⁴ 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 was corroborated by the increase of TOC during the reaction caused by the partial release of the polyphenols from ^BT2Fe (Table S5, ^SI section), adjusting the pH to 3.4.

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 H₂O₂ presented negligible adsorption capacity of the nanomaterials with less than 4% of adsorption. Moreover, the reaction with MO and H₂O₂ 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, H₂O₂ can accelerate the process. The iron leaching was determined by FAAS, in the supernatant of an aqueous suspension of ^BT1Fe or ^BT1Fe₉₀₀ in presence of H₂O₂ at the same concentration of the MO degradation tests. ^BT1Fe suffered 53% of iron loss during the reaction, representing homogeneous iron concentration of 20.25 mg L^–1, while for ^BT1Fe₉₀₀ 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 Fe₂O₃, and represented 7.0 wt.% of iron for ^BT1Fe (Table S2). Iron leaching was also observed for iron oxide nanomaterials prepared with Yerba mate and black tea extracts anchored in amino-functionalized SiO₂, representing the main drawback of these green nanomaterials.⁶

To account for the homogeneous contribution from the iron leaching of the catalyst during the Fenton degradation, a control test using FeCl₃·6H₂O 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 H₂O₂.²

The time trace of the azo band decay for the most efficient catalysts ^BTnFe were fitted with first and second order kinetic models. Figure 8 shows the decrease of the dye over time for ^BT2Fe, which the best fit was obtained for pseudo-first order exponential decay shown in equation 2, leading to higher R² 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 ^BT1Fe and ^BT3Fe are presented in Figures S13 and S15 (^SI section). The kinetic data and degradation extension are summarized in Table 5. ^BT3Fe 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 ^BTnFe catalysts are in the range of published Fenton-like system in degradation of lincomycin (LCM) Fe⁰/H₂O₂ (k_obs = 0.0022 min^-1),¹ as well as of green FeONPs prepared from tea polyphenols added to Fe0 for LCM degradation Fe⁰/polyphenol/H₂O₂ (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

Figure 8
Kinetic experimental data and first- and second-order non-linear fit for MO (4.7 × 10^-5 mol L^-1) decolorization at 25 ºC by a ^BT2Fe (10.0 mg).

Table 5
Kinetic data for pseudo-first order equation determined by the non-linear fitting, and degradation of the methyl orange

Catalyst	kobs / min-1	R2	Decolorization300min / %
BT1Fe	0.0100	0.932	94
BT2Fe	0.0100	0.965	91
BT3Fe	0.0122	0.969	95

• k_obs: first-order rate constant; R²: correlation coefficient.

(2) ${\text{C}}_{\text{t}}={\text{C}}_0-{\text{e}}^{-{\text{k}}_{\text{obs}}\text{t}}$

where C₀ is the concentration of MO at time = 0, C_t is the concentration of MO at given time (t), and k_obs is the first-order rate constant.

All annealed catalysts ^BTnFe₉₀₀ 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 ^BT2Fe₉₀₀.⁴¹ These 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.

Figure 9
Degradation of methyl orange (4.7 × 10^-5 mol L^-1) at 25 ºC catalyzed by 10.0 mg of catalyst: (a) electronic UV-Vis spectra over time for ^BT2Fe₉₀₀, and (b) degradation percentage for all catalysts.

Comparing the annealed materials, ^BT1Fe₉₀₀ was the most active with 26%, and presented Fe₃PO₇ as the major phase.

In the other hand, ^BT2Fe₉₀₀ and ^BT3Fe₉₀₀ presented α-Fe₂O₃ 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.⁴² The material ^BT2Fe₉₀₀ 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.⁴² Furthermore, ^BT2Fe₉₀₀ also presented the highest content of α-Fe₂O₃ 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).

Conclusions

Green iron oxides nanomaterials were prepared using Camellia sinensis (black tea) extracts of three different regions. Characterization of the as-prepared materials showed the formation of FeOOH, as well as Fe^III/Fe^II polyphenols complexes indicating chelation and partial reduction of the Fe^III precursor to Fe^II. Then, the materials were thermally treated at 900 °C and produced α-Fe₂O₃, FePO₄ and Fe₃PO₇ phases. The origin of the black led to reproducible materials in terms of phase composition, but with different proportions among the crystalline phases. Furthermore, the phosphorous element come from the plant to form the phosphates. The degradation of the methyl orange dye followed first-order kinetics and was more efficient with the as-prepared materials because of the pH lowering from the polyphenol proton release during the Fenton reaction, adjusting naturally the pH to around 3 that is the optimum pH for Fenton system. In conclusion, this work show that iron crystalline nanomaterials can be formed independent of the black tea origin, using the polyphenols as biotemplates. Although these materials are interesting for the materials point of view, in the Fenton reaction the polyphenols present in the as-prepared materials are important to regulate the pH and led to complete decolorization of methyl orange.

• #
  Present address: GSK Biopharma Production Dept. 9910 Belward Campus Dr, Rockville, MD 20850 USA

Acknowledgments

This work was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq (PQ-2/2018), Fundação Carlos Chagas de Amparo à Pesquisa do Estado do Rio de Janeiro - FAPERJ [JCNE: E-26/203.023/2018; Grupos Emergentes: E-26/010.002171/2019); Undergraduated Scholarship: E-26/202.126/2017; Post-doctoral scholarship PDJ10: E-26/290.125/2020] and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) (Finance Code 001).We acknowledge DFQ/IQ/UFRJ for the use of S8-Tiger WD FRX spectrometer (FAPERJ), LABNANO/ CBPF (Rio de Janeiro) for TEM and SEM measurements in the JEM 2100F Jeol and SEM-FEG 7100FT, respectively. We acknowledge RECAT-UFF for XRD analyses, LCM/CAIPE-UFF for the Raman analyses, and LACES/UFRJ for the thermogravimetric analyses.

Supplementary Information

Supplementary information (black tea characterization; iron nanoparticles characterization: SEM, EDS, TG, XRD Rietveld refinement, FTIR; methyl degradation data, UV-Vis spectra and kinetic plots) is available free of charge at http://jbcs.sbq.org.br as PDF file.

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Edited by

Editor handled this article: Izaura C. N. Diógenes (Associate)

Publication Dates

• Publication in this collection
  01 May 2023
• Date of issue
  May 2023

History

• Received
  31 July 2022
• Accepted
  10 Nov 2022