Doping TiO₂ with Fe from iron rusty waste for enhancing its activity under visible light in the Congo red dye photodegradation

Abstract

An afford to enhance TiO₂ activity under visible light as well as to utilize the iron rusty waste, has been conducted by doping Fe from the waste into TiO₂. The doping was performed by sol-gel method of titania tetra isopropoxide with Fe³⁺ ions dissolved from the iron rust waste. In the doping, the concentration of Fe³⁺ was varied giving various mole ratios of TiO₂:Fe. The doped TiO₂ photocatalysts were characterized using FTIR, XRD, SRUV, and SEM-EDX instruments. The photocatalytic activity of the doped TiO₂ was evaluated by photodegradation of Congo red under visible light. The effect of some parameters that govern the photodegradation process such as the amount of Fe dopant, reaction time, photocatalyst mass, solution pH, and initial concentration of dye was also studied. The characterization results reveal that Fe³⁺ ions from the rusty waste have been doped into TiO₂ which can remarkably narrow the band gap energy (Eg), shifting into the visible zone. In accordance, the activity of TiO₂ under visible light in the dye photodegradation is considerably enhanced. The Eg decreasing and actively improving the doped TiO₂ are controlled by the amount of Fe dopant, and the most effective Eg decreasing is shown by TiO₂–Fe (1:0.8), but the highest activity is observed for TiO₂–Fe (1:0.4). It is also found that the highest photodegradation of Congo red 5 mg/L in 50 mL of the solution over TiO₂–Fe (1:0.4) under visible light, that is about 99%, can be reached by applying 60 mg of the photocatalyst mass, in 60 min, and solution pH 5. It is implied that the rusty waste can be utilized to prepare the visible responsive photocatalyst that can be used for preventing dye pollution.

Introduction

TiO₂ nanoparticles have attracted considerable attention as a photocatalyst for the degradation of various organic pollutants [1,2,3,4]. Due to being stable, inexpensive, having high photocatalytic activity, nontoxicity, and electron transfer to molecular oxygen, TiO₂ has turned into one of the most popular photocatalysts [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. Unfortunately, TiO₂ is a semiconductor with a large band gap energy (Eg) that is 3.2 eV for the anatase phase, which is equal to a wavelength of 387 nm, allowing it to have high activity only under UV light or be less active under visible light [5, 7,8,9,10,11,12,13,14,15]. This confines the application of TiO₂ under sunlight because sunlight only contains about 3–5% UV light and dominantly consists of visible light [5, 7]. Doping TiO₂ with transition metals is found as an intensive afford to extend and enhance TiO₂ activity under visible light [5, 7,8,9,10,11,12,13,14,15]. Among the transition metals, Fe dopant is reported to show a significant effect in improving TiO₂ activity [9,10,11,12,13,14,15]. The Fe³⁺ ion has a smaller ionic radius (0.64 Å) and electronic charge (3+) than that of Ti⁴⁺ (ionic radius = 0.68 Å with 4+ charge), enabling Fe³⁺ to easily substitute the Ti⁴⁺ ion from Ti–O–Ti [10, 11]. This substitution can create a new band occupying the conduction and valence bands, which remarkably narrows the gap [9,10,11,12,13,14,15].

In the Fe doping, the sources of Fe dopants frequently used are commercial pure salts such as Fe(NO₃)₃ [9,10,11,12,13,14,15], FeSO₄ [13], and K₃Fe(CN)₆ [12], that have a high price, leading to the doping process costly. Hence, finding a low-cost Fe source is beneficial, which can be obtained from iron rusty waste found easily in the environment. Rusted iron is an iron that undergoes oxidation by oxygen air and water, to form Fe₂O₃ [16,17,18]. This rust formation causes the iron material brittle [16], so it losses its function, and finally the material becomes useless solid waste. The presence of iron rusty waste in the environment can reduce aesthetics and potentially cause tetanus of the open hurt [18]. In fact, the iron rust waste with high Fe content can be utilized as a source of Fe dopant into TiO₂ resulting in the visible responsive photocatalyst. So far, doping TiO₂ with Fe from iron rusty waste is untraceable.

In this present work, the use of iron rust waste as a source of Fe dopant for enhancing the TiO₂ activity under visible light is systematically addressed. The effect of Fe doping on the photocatalyst performance under visible light was evaluated through Congo red dye photodegradation by laboratory experimental. The Congo red dye (C₃₂H₂₂N₆Na₂O₆S₂), as seen in Fig. 1, is sorted as a pollutant model due to its widely used in the textile and other wearable industries [19,20,21,22], and biochemical laboratories [23]. The intensive use of the dye must release a large volume of wastewater containing high dye levels. The Congo red dye contamination adheres to the ecosystem and human health, since it is able to destroy biotics as well as causes nausea, vomiting, and diarrhea [19,20,21,22], event induces cancer for humans. Various methods have been reported for diminishing the dye such as adsorption [19], biodegradation [20], ozonation [21], photo-Fenton [22, 23], and photocatalytic degradation over Fe-doped TiO₂ [24]. However, photodegradation over TiO₂ doped with Fe from the iron rusty waste has not been explored yet.

Fig. 1

Chemical structure of Congo red

In the photodegradation of the Congo red dye over the Fe-doped TiO₂ photocatalyst, various variables such as the amount of Fe doping concentration, pH of the dye solution, photocatalyst mass, contact time, and initial concentration of the Congo red dye are optimized to obtain the best condition of the dye removal.

Experimental

Material

The materials used in this study were Titanium (IV) isopropoxide (TTIP) 97%, ethanol (C₂H₅OH) 95%, hydrogen chloride (HCl) 35%, nitric acid (HNO₃) 68%, sodium hydroxide (NaOH) with the pro analysis (p.a) grade that was used as received. In addition, iron rusty solid waste taken from around Yogyakarta City was also employed.

Instruments

Visible light (Philips 36 W) and UV light lamps (Philips 160 W) were used as the light source for the photocatalysis process. The instruments operated for analysis and characterization included UV-visible spectrophotometer (Analytic Jena), specular reflectance UV-visible spectrophotometer (SR UV-Vis, Shimadzu UV-1700 Pharma Spec), Fourier-transform infrared spectrophotometer (FT-IR) (Shimadzu Prestige-21), X-ray diffraction (XRD, Rigaku Multibox 2 kW), and scanning electron microscope with energy dispersive X-ray spectrophotometer (SEM-EDX, Phenom-World).

Methods

Preparation of the Fe-doped TiO₂

Doping TiO₂ with Fe atoms was conducted by using sol-gel method, that was by interacting the TTIP solution with the solution containing Fe³⁺ dissolved from the iron rusty waste. The Fe³⁺ solution was prepared by dissolving 0.80 grams of the iron rusty waste in 7.5 ml of aqua regia (a mixture of concentrated HCl and HNO₃ with volume ratio of 3:1) to form a brown clear solution indicating the presence of Fe³⁺ ions, then the solution was diluted into 100 mL. The concentration of Fe³⁺ in the solution determined by using AAS was found as much as 0.05 mmole/mL.

At the same time, 10 ml of TTIP 1 mole/L was dissolved in 30 mL of ethanol while being stirred magnetically for 15 min to get a clear solution. Five beaker glasses containing 10 mmole of the TTIP were added with 1, 2, 4, 10, and 20 mL respectively, and were continued by the addition of acetic acid to get pH 3 along with magnetic stirring for 3 h to get a clear homogenous solution. Then the clear solutions were cooled in the refrigerator for 24 h so that the gel phase was produced. The gels were calcined at 500 ^oC for 3 h, and so the powder samples were obtained as the Fe-doped TiO₂ photocatalysts. The doped photocatalysts obtained were notified as TiO₂–Fe (1:0.05), TiO₂–Fe (1:0.1), TiO₂–Fe (1:0.2), TiO₂–Fe (1:0.5), and TiO₂-Fe (1:1) following the mole ratio of Ti to Fe.

Characterization

The doped-TiO₂ photocatalyst powders were characterized by using several instruments including SR-UV/Visible, FTIR, XRD spectrometers, and SEM-EDX. The SR-UV/Visible spectra of the samples were taken from 200 to 800 nm of the wavelength. The FTIR spectra were scanned in the range of 4000–400 cm⁻¹ of the wavenumbers. The XRD patterns were recorded on the instrument using Cu-Kα as an X-ray source, from 3 to 50^o of the two tetha. The SEM images along with the EDX spectra were shot from the microscope with × 10,000 magnification.

Photoactivity assessment

In this typical, a series of 6 dye solutions as much as 50 mL having 10 mg/L of the dye concentration at pH 5, were added with 40 mg of the un-doped, and the doped TiO₂ photocatalysts with various Fe amounts. The mixtures were placed in a closed box equipped with visible lamps and then were irradiated with the visible light accompanied by stirring magnetically for 60 min. After the desired time, the mixtures were filtered to get the clear filtrates of the LAS solutions. The clear solutions were analyzed by using a UV-visible spectrophotometer based on the reaction with methylene blue and extraction with chloroform solvent. The blue solution absorbance was then measured at 499 nm. The absorbance observed then was interpolated into a respective standard curve to get the dye concentration left in the solution. The amount of dye degradation presented in % was determined by the following relationship:

$$E=\frac{C_0-{C}_f}{C_0}\times 100\%$$

E = the amount of dye degraded (%)C_o = initial amount of the dye (mg)C_f = the amount of dye undegraded or left in the solution after photodegradation (mg)

The same procedure was repeated with different conditions, including irradiation time (5, 15, 30, 45, 60, 75, 90, and 120 min), photocatalyst weight (10, 20, 30, 40, 60, 80, and 100 mg), solution pH (1, 3, 4, 5, 7, and 9), as well as the dye initial concentration (5, 10, 25, and 50 mg/L), where the volume of the dye solution was set to be 50 mL. When one variable was varied, the other variables were kept to be constant.

Result and discussion

Characterization results

SR-UV/visible data

The SR-UV/visible spectra of the photocatalysts are illustrated in Fig. 2. It is seen in the figure, that the Fe-doped TiO₂ samples show their absorption edge at the longer wavelengths (λ), entering the visible region than the absorption of the undoped one. This absorption shifts allowing the doped TiO₂ to strongly absorb the visible light, which is hoped to show higher activity in the presence of visible light.

Fig. 2

The SR-UV/Visible spectra of a TiO₂, b TiO₂–Fe(1:0.05), c TiO₂–Fe(1:0.1), d TiO₂–Fe (1:0.2), e TiO₂–Fe(1:0.5), and f TiO₂–Fe (1:1)

The wavelengths of the absorption edge of all samples were presented in Table 1, along with the band gap energy (Eg) values that were calculated by the Tauc plot method as seen in Fig. 3. The data in Table 1 illustrates that Fe doping results in a decrease in the band gap energy (Eg) of TiO₂ from 3.15 eV to 3.05–2.38 eV. With the higher amount of Fe dopant, the lower Eg values are observed. Similar data have also been reported previously [5,6,7,8,9,10,11,12,13,14,15]. The Eg reduction is caused by the narrowing gap in the TiO₂ semiconductor structure due to the new band formation located between the valence and conduction bands. The new band generated by the dopant clearly notifies that the Fe³⁺ in the solution has been successfully doped [14]. Generally, the remarkable Eg decrease can also imply that the Fe doping into the TiO₂ structure follows the interstitial mechanism [15].

Table 1 Band gap (Eg) energy values of the TiO₂ and TiO₂–Fe

Fig. 3

The Tauc Plot of a TiO₂, b TiO₂–Fe(1:0.05), c TiO₂–Fe(1:0.1), d TiO₂–Fe (1:0.2), e TiO₂–Fe(1:0.5), and f TiO₂-Fe (1:1)

FTIR data

In Fig. 4, the FTIR spectra of the Fe-doped TiO₂ photocatalysts are seen similar to the spectra of the undoped one. The appearance of the peaks at the wavenumbers of 3749–3425 cm⁻¹ and 1635 cm⁻¹, assign respectively the stretching and bending vibrations of O–H from H₂O adsorbed on the surface of the photocatalyst [14]. The characteristic peak of TiO₂ appears in the wavenumber of 400–800 cm⁻¹, representing the Ti–O–Ti or O–Ti–O vibrations [5, 14].

Fig. 4

FTIR spectra of a TiO₂, b TiO₂–Fe(1:0.05), c TiO₂–Fe(1:0.1), d TiO₂–Fe(1:0.2), e TiO₂–Fe(1:0.5), and f TiO₂–Fe(1 :1)

In the spectra of all TiO₂–Fe samples, peaks at around 450–493 cm⁻¹ can be seen, which are the shifts from the peak of 509 cm⁻¹ belonging to the undoped TiO₂. The shifts can indicate the disturbance of the Ti–O–Ti bond by the Fe dopant, due to the interaction between oxygen atoms in the Ti–O–Ti bond with the Fe doped [14]. Such interaction indicates the occurrence of the interstitial doping mechanism [14, 15].

XRD data

The XRD patterns of the undoped TiO₂ and all of the TiO₂–Fe photocatalysts were displayed in Fig. 5. The characteristic peaks of TiO₂ appear at the 2θ angles of 25.18°; 38.26°; 47.7°; 54.44°; 55.26°; 62.52°, which good match with TiO₂ anatase crystal data as listed in COD CIF File No.: 00-152-6931 with the Miller index (hkl) to (101), (112), (200), (105), (211), and (213) respectively [5, 12].

Fig. 5

X-ray diffraction patterns of a Fe₂O₃, b TiO₂, c TiO₂–Fe(1:0.05), d TiO₂–Fe(1:0.1), e TiO₂–Fe(1:0.2), f TiO₂–Fe(1:0.5), and g TiO₂–Fe(1:1)

Further, it is clearly observed that all TiO₂-Fe samples with various Fe content have similar patterns as the undoped TiO₂ pattern, but with lower intensities and without any new pattern of the dopant. This finding suggests that Fe doping gives no effect on the crystal phase change, as also reported previously [9, 13,14,15]. The lower intensities refer that the doped TiO₂ is in the less crystalline phase, due to the inhibition of the TiO₂ crystallinity growth [15]. Such inhibition of the crystallographic growth is most probably affected by the Fe dopant inserted in the TiO₂ crystal lattice, which was well documented in previous works [15]. Then, the absence of the dopant pattern implies that the Fe dopant has been successfully incorporated into the crystal lattice of TiO₂ due to the nearly identical ionic radius to that of the Ti⁴⁺ cation (0.0745 nm) [15]. In addition, the data in Table 2 demonstrates that the Fe doping has dismissed the average crystal sizes of TiO₂, and the gradual size reduction is observed as the amount of the dopant is increased. The decreasing crystal size due to the Fe doping may indicate that the doping takes place through the substitution of the Ti atom in Ti–O–Ti by Fe dopant possessing smaller ionic radii (0.64 A) than that of Ti (0.68) [15]. This reason is clearly supported by the fact that more amount of Fe dopant in TiO₂–Fe strongly depresses the crystallite size.

Table 2 Effect of Fe doping on the average crystallite size of TiO₂

TEM images

The TEM image of undoped TiO₂ shows cleaner spheres, while Fe-doped TiO₂ appears as darker-colored spheres, as illustrated by Fig. 6. The dark color in the TEM image illustrates the presence of Fe dopant atoms that have successfully entered the crystal lattice of TiO₂. Other Authors have also reported similar images [14].

Fig. 6

The TEM images of a TiO₂ and b TiO₂ –Fe 1:0.5

The activity of the Fe- doped TiO₂

Effect of the dopant amount

The effect of Fe doping on the CR dye photodegradation is illustrated in Fig. 7. It is seen in the figure that Fe doping has considerably enhanced the TiO₂ activity in dye degradation, both under UV and visible light. Moreover, as the Fe dopant amount is enlarged, the higher activity is notified, but the opposite data is observed for the further increase of the Fe amount.

Fig. 7

The dye photodegradation over: 1 TiO₂, 2 TiO₂–Fe(1:0.05), 3 TiO₂–Fe(1:0.1), 4 TiO₂–Fe(1:0.2), 5 TiO₂–Fe(1:0.5), and 6 TiO₂–Fe(1:1) under visible and UV lights. (Dye concentration = 10 mg/L, dye solution volume = 50 mL photocatalyst weight = 40 mg, irradiation time = 60 min, and pH = 5)

The enhancement of the dye photodegradation under visible light, over the undoped TiO₂, is induced by the larger ability of the doped TiO₂ in the visible absorption due to their lower Eg corresponding to visible photon energy. With such Eg values, TiO₂ can be activated by visible light to form holes and electrons. The holes then are scavenged by H₂O molecules that become OH radicals, while the electrons can be captured by dissolved oxygen to form superoxide (^.O₂) radicals. These radicals act as strong oxidizing agents that are responsible for dye degradation [5]. The reactions of radical formation and dye degradation are presented below:

$${\textrm{TiO}}_2+ hv\rightarrow {\textrm{TiO}}_2\ \left({e}^{-}+{h}^{+}\right)$$

(1)

$${h}^{+}+{\textrm{H}}_2\textrm{O}\rightarrow {\textrm{H}}^{+}+\bullet \textrm{OH}$$

(2)

$${e}^{-}+{\textrm{O}}_2\rightarrow \bullet {\textrm{O}}_2$$

(3)

$${\textrm{C}}_{32}{\textrm{H}}_{22}{\textrm{N}}_6{\textrm{Na}}_2{\textrm{O}}_6{\textrm{S}}_2+\bullet \textrm{OH}\rightarrow {\textrm{CO}}_2+{\textrm{H}}_2\textrm{O}+{\textrm{Na}}_2{\textrm{SO}}_4+{\textrm{HNO}}_3$$

(4)

As the Fe amount increases, the larger visible light can be absorbed, which produces more radicals, which is a very conducive condition for dye photodegradation. The enhancement of the degradation is advanced to proceed up to reach the maximum level. However, the amount of Fe exceeding the optimum level leads to the degradation being dismissed. A large amount of Fe dopant may form the aggregate with a larger size covering the active surface of TiO₂, which prevents the direct contact of light with the photocatalyst. As a consequence, the less OH radicals can be produced.

The activity of the doped-TiO₂ under UV light is also found to be higher than that of the undoped photocatalyst. The presence of the Fe dopant in the structure of the TiO₂ crystal creating a new band can capture the electrons excited from the valence band [15]. Consequently, the recombination of holes and electrons can be delayed, so that more holes and radicals are available. This situation can promote more effective dye degradation. Further enhancement of the degradation is notified as the number of dopants increases, which should be resulted from the greater prevention of the recombination. Unfortunately, the TiO₂–Fe(1:1) with the highest fraction of the Fe dopant, possessing a very narrow gap, as indicated by the lowest Eg, can facilitate the electrons and holes to recombine. In other words, a very large dopant amount in TiO₂ can act as a recombination center. Hence, in the degradation process is a lack of holes and OH radicals, giving low degradation.

Effect of the irradiation time

It is demonstrated in Fig. 8, that the improvement of the dye photodegradation can be resulted as the irradiation is extended from 5 to 60 min, but the longer time than 60 min gives no effect on the degradation. Prolonged irradiation time enables more effective contact between the visible light with the photocatalyst, resulting in a larger number of holes and OH radicals. In addition, with a longer time, the contact between the OH radicals with the dye can intensively occur. These situations are conducive to the dye degradation process. After 60 min running, the photocatalyst may be saturated that is not able to release more OH radicals, providing the same radicals quantity. This explains why the photodegradation remains the same although the irradiation time is extended up to 120 min.

Fig. 8

Effect of the irradiation time on the dye photodegradation over TiO₂–Fe (1:0.5) under visible light. (Dye concentration = 10 mg/L, dye solution volume = 50 mL photocatalyst weight = 40 mg, and pH = 5)

Effect of the photocatalyst mass

Figure 9 displays the dye photodegradation resulted from the process using various masses of the photocatalysts under visible exposure. It is notified that the dye degradation sharply increases with the increase of photocatalyst masses, due to the larger number of OH radicals provided by more quantity of the photocatalyst. The contradiction trend is observed when the photocatalyst mass was further enlarged. The excessive amount of the photocatalyst creates higher turbidity in the dye solution, which hinders visible light penetration. Such less penetrated light may prevent the TiO₂ to devote more OH radicals, and so the lesser effective degradation is obtained.

Fig. 9

Effect of the photocatalyst mass on the dye photodegradation over TiO₂–Fe (1:0.5) under visible light. (Dye concentration = 10 mg/L, dye solution volume = 50 mL irradiation time = 60 min, and pH = 5)

Effect of the solution pH

The trend of the dye photodegradation results at the pH alteration was displayed in Fig. 10. The solution pH determines the charges of both TiO₂ and Congo red dye. The pH of the zero point charge of TiO₂ is reported as 6.5, where TiO₂ is in the neutral charge. It is assigned that at lower pH than 6.5, TiO₂ has a positive charge, while at pH higher than 6.5, the positive charge is formed on the TiO₂ surface [1, 2]. Congo red dye has an isoelectric charge at pH 3, suggesting that at pH lower than 3, the dye has a positive charge, and it will have a negative charge at pH higher than 3 [19].

Fig. 10

Effect of solution pH (dye concentration = 10 mg/L, dye solution volume = 50 mL photocatalyst weight = 40 mg, and irradiation time = 60 min)

From Fig. 9, it can be seen that at very low pH, the dye degradation is less effective. In this condition (pH 1), both TiO₂ and the dye have positive charges resulting from the protonation by a large number of H⁺ ions. Consequently, there is a repulsion effect for the interaction between TiO₂ and the dye, giving low adsorption of the dye on the TiO₂ surface. Besides, in the protonated form, TiO₂H⁺ is confined to release electrons and holes, which further reduces the OH radicals production. These conditions are less conducive to dye degradation.

When the pH is increased to 3, the degradation also rises. In this pH, the dye has no charges or is in the neutral species, and the TiO₂ is kept to be protonated, allowing TiO₂ to adsorb the dye but must be in the small portion. By elevating the media pH from 3 to 5, the protonated TiO₂ particles are still present in the large number, while the dye molecules are dominantly in the negative charge. Thus, the dye adsorption and so dye degradation takes place effectively, and even reach the maximum degradation. It is observed that at higher pH than 5, the dye degradation gradually decreases. In the media with pH 7–9, both TiO₂ and the dye are negatively charged, causing a repulsive effect in the dye adsorption on the TiO₂ surface. Accordingly, it is reasonable therefore that the low dye degradation is the result.

Effect of the initial dye concentration

As appeared in Fig. 11, the dye degradation gradually declines when the dye’s initial concentration is gradually elevated. The complete dye degradation is only observed for 5 mg/L of the concentration, and with 10 mg/L, the dye degradation is insignificantly different from the 5 mg/L. The photocatalyst is found to be less effective in dye degradation with higher dye concentrations. In that situation, the OH radicals from the same photocatalyst masses, are constant, while the dye molecules are in higher amounts, leading to a decrease in the dye photodegradation [25].

Fig. 11

Effect of the initial dye concentration (dye dye solution volume = 50 mL photocatalyst weight = 40 mg, irradiation time = 60 min, and media pH = 5)

Stability test of the TiO₂–Fe

The stability of the TiO₂–Fe has been checked by measuring the concentration of the Fe dissolved from the photocatalyst during the dye photocatalytic degradation process. The photocatalyst tested was TiO₂–Fe (1:0.5) showing the best performance, and the results are presented in Fig. 12. It is seen in the figure that during the photocatalysis degradation, a very low amount of Fe dissolved is detected. The longer the irradiation time, the Fe dissolved is slightly increased. It is believed hence that the Fe dopant has high stability.

Fig. 12

The amount of the Fe dopant dissolved from the doped photocatalyst during the photocatalysis degradation process

Conclusions

It can be concluded that the Fe³⁺ ions dissolved from the iron rusty waste have been successfully doped into TiO₂ structure, which can shift the light absorption into longer wavelengths, and so decreasing the band gap energy (Eg) values, that are categorized in the visible regime. The Eg narrowing is proportional to the amount of Fe dopant, and the most effective Eg reduction is demonstrated by the highest Fe content in the doped-TiO₂ that is TiO₂–Fe (1:1). The Eg decreasing results in the enhanced TiO₂ activity under visible light in the Congo red photodegradation, and the highest activity is found in the TiO₂–Fe (1:0.5). The best condition for the degradation of the 10 mg/L dye concentration in 50 mL solution over TiO₂–Fe (1:0.5) is achieved by using 40 mg of the photocatalyst mass, in 60 min, and at pH 5, that is 99 %. It is clearly inferred that the rusty waste can be utilized for preparing the visible responsive TiO₂ in order to prevent Congo red dye pollution.