Efficient degradation of amoxicillin using Bi₂O₃/Fe synthesized by microwave-assisted precipitation method

Abstract

Bi₂O₃/Fe material has been successfully synthesized using the microwave-assisted precipitation method. This study aims to analyze the effect of Fe on the characteristics of Bi₂O₃ and its ability to photodegrade amoxicillin. XRD characterization showed that all samples (Bi₂O₃/Fe 0–9%) formed an α-Bi₂O₃ phase. The addition of Fe concentration did not significantly change the crystal structure of α-Bi₂O₃. The morphology of Bi₂O₃/Fe is mostly rod and spherical porous which is indicated as Bi₂O₃ and Fe particles. EDX characterization informed that all samples contained Bi, O, and Fe elements which were evenly distributed. The addition of Fe up to 3% will increase the crystallite size and specific surface area of Bi₂O₃ but decrease the energy gap. Meanwhile, the addition of more than 3% Fe causes an increase in the energy gap of the Bi₂O₃ but will decrease the crystallite size and specific surface area. Bi₂O₃/Fe 3% is the best sample because it has the largest crystallite size (29.1 nm), the lowest energy gap (2.02 eV), the largest specific surface area (6.864 m²/g), and can degrade amoxicillin to 76.34% with a degradation rate of 0.0079 min⁻¹.

Keywords:

• Bi₂O₃/Fe
• microwave irradiation
• photodegradation
• amoxicillin

PUBLIC INTEREST STATEMENT

Amoxicillin is one of the most used antibiotics. The use antibiotics of amoxicillin which is quite a lot in daily life in addition to providing health benefits, also make one source of pollution of the water environment through medical waste. The existence of amoxicillin in the water environment can damage the environment and threaten health. Photocatalyst material currently has an important role in the degradation of medical waste. In this study, we found effective and efficient material for the degradation of amoxicillin medical waste, namely Bi₂O₃/Fe which was synthesized using the precipitation method assisted by microwave radiation. It is hoped that this discovery is useful for overcoming water environmental problems by Amoxicillin medical waste.

1. Introduction

In recent years, environmental pollution from liquid medical waste has become increasingly widespread. It is a challenge for both researchers and the pharmaceutical industry because drugs produce medical waste that can threaten ecosystems and human health (Wang et al., 2019). One of the medical waste that is a source of environmental pollution is amoxicillin. Amoxicillin (AMX) is the most widely used type of antibiotic because of its extraordinary antibacterial properties (Jung et al., 2012). However, AMX has low biodegradability and remarkable stability with many complex aromatics in its molecule, making it difficult to destroy by conventional methods (Zhang et al., 2019). Therefore, we need the proper technique to degrade AMX efficiently.

Several alternatives have been carried out to remove these compounds, including reverse osmosis, adsorption, and advanced oxidation technologies such as fenton reaction, ozonation, and photocatalytic technology (Benitez et al., 2011). Photocatalytic shows promising prospects among these methods due to its sustainability and environmental friendliness (Wang et al., 2018). Bismuth oxide (Bi₂O₃) is a photocatalyst material that is of interest to researchers because of its high redox inversion, environmental friendliness, thermal stability, and energy bandgap between 2.58 and 2.85 eV, which shows a response to visible light (Basith et al., 2018). The phase (monoclinic) Bi₂O₃ has semiconductor properties, is stable at low temperatures, and is more energy-efficient in the synthesis process (Gadhi et al., 2016). However, photon-induced electron–hole pairs have poor light utilization and fast recombination rates limiting photocatalytic activity (Zhou et al., 2019).

In this study, the Bi₂O₃ semiconductor will be inserted with Fe material with the microwave-assisted precipitation method. This experiment is believed to increase the rate of electron–hole pair separation by acting as a shallow trap where the permissible energy states are close to the conduction band and valence band and reduce the rate of electron–hole pair recombination in Bi₂O₃ materials (Zhu et al., 2011). Finally, the authors hope to be able to produce innovative materials that are effective in degrading AMX.

2. Experiment method

2.1. Synthesis of Bi₂O₃/Fe

Bi₂O₃/Fe was synthesized using a microwave-assisted precipitation method. The synthesis process of Bi₂O₃/Fe is shown in Figure Figure 1. Bi(NO₃)₃.5H₂O 0.5 g and Fe(NO₃)₃.9H₂O with different mass (1/100, 3/100, 5/100, 7/100, 9/100) (%wt) were dissolved in 50 ml of 5% HNO₃ solution (1 mol/l) stirred for 10 minutes using a magnetic stirrer on a hot plate at room temperature. A total of 250 ml of NaOH (1 mol/l) was added to the solution and stirred for 2 hours, then allowed to stand to produce a precipitate. The precipitate was separated and heated on a hotplate for 2 hours at 120°C to produce a still moist Bi₂O₃/Fe powder. Furthermore, the resulting Bi₂O₃/Fe powder was then dried by exposing it to 100 Watt microwave power for 3 hours.

Figure 1. The synthesis process of Bi₂O₃/Fe.

2.2. Characterization

Structural analysis of Bi₂O₃/Fe powder was carried out using X-Ray Diffraction (XRD Shimadzu 6100/7000 type). The X-ray wavelength used is 1.5406 Å. Data retrieval is carried out every 0.02° angle range. The crystallite size of Bi₂O₃/Fe was calculated using the Debbye-Scherrer equation (Equation 1(1) $Ds=\frac{k\lambda }{\beta cos\theta }$(1) ; Xie et al., 2019).

(1) $Ds=\frac{k\lambda }{\beta cos\theta }$(1)

Ds is the crystallite size (nm), $\lambda =$ 1.5406 Å is the wavelength of X-rays, k = 0.9 is the Scherrer constant, is the Full Width and Half Maximum or FWHM (in radians), and is the Bragg angle of diffraction or peak position (in degree).

The surface morphology of Bi₂O₃ powder was obtained from the Scanning Electron Microscopy and Energy Dispersive X-Ray (SEM-EDX) characterization data. The SEM-EDX image is used to determine the element, shape, and particle size. The specific surface area was obtained from the Surface Area Analyzer (SAA) characterization. UV-Vis transmittance data of Bi₂O₃/Fe powder was used to determine the energy band gap value.

2.3. Amoxicillin degradation

The degradation was carried out by adding 0.02 g of Bi₂O₃/Fe powder into 50 ml of 10 ppm AMX solution obtained by dissolving 10 mg of AMX powder (Amoxicillin Trihydrate 500 mg generic) into 1 liter of distilled water. The solution was irradiated with UV light for 180 minutes, and the degradation products were taken every 30 minutes. The degradation results were analyzed using a UV-Vis Spectrophotometer UH5300 (Hitachi, Japan). Photocatalyst efficiency (Ef) is calculated using Equation 2(2) $\mathrm{E}\mathrm{f}\left(\mathrm{\%}\right)=\left(1-\frac{C_t}{C_0}\right)x100\mathrm{\%}$(2) . (Sudrajat et al., 2018).

(2) $\mathrm{E}\mathrm{f}\left(\mathrm{\%}\right)=\left(1-\frac{C_t}{C_0}\right)x100\mathrm{\%}$(2)

where C₀ and C_t are the initial and final concentrations of the solution.

3. Results and disscusions

Bi₂O₃/Fe has been synthesized using a microwave-assisted precipitation method. This method is very advantageous because it is energy-intensive, homogeneous, efficient, can reach high temperatures, and start reactions quickly. Microwaves simplify and speed up the synthesis process by generating heat in a short time due to the activity of compounds that oscillate and collide with each other. Simultaneously, microwaves help the growth (nucleation) of the material due to the slight gradient or heat difference that occurs (Fatimah et al., 2015). The high-frequency motion of molecules from microwaves causes the magnetic dipoles of the molecules to change direction very quickly and cause friction. As a result, friction will generate thermal energy used in the sintering process to form a photocatalyst Bi₂O₃/Fe. In addition, molecular vibrations due to microwaves cause chemical bonds in the precursor to be rearranged (Rahmayanti et al., 2015).

The synthesized powder shows a yellow physical color that shifts brownish with increasing Fe concentration, as shown in Figure Figure 2. The color change occurs because Fe(NO₃)₃.9H₂O when dissolved, forms a brownish to dark-colored solution due to hydrolysis. As the concentration of Fe increases, the reaction shifts in the opposite direction of the substance. Bi₂O₃ added with Fe will shift its color from yellow to brown. It follows Le Chatelier’s Principle, which studies changing conditions on an equilibrium system. If colored ions are added to a reaction, the compound formed will be darker (Prakoso et al., 2012).

Figure 2. Physical appearance of the sample Bi₂O₃/Fe: (a) 0% (b) 1% (c) 3% (d) 5% (e) 7% (f) 9%.

To confirm the success of Bi₂O₃/Fe synthesis, the first thing to do is test the crystallinity. Based on the XRD Bi₂O₃/Fe pattern as shown in Figure Figure 3, the resulting diffraction peaks correspond to JCPDS No. 27–0053 with space group P21/c, monoclinic crystal system, unit cell parameters; a = 5.83 Å b = 8.14 Å c = 7.48 Å β = 67.07°, and density 9.467 g/cm³ (Devi et al., 2019). The main diffraction peak is at 2θ = 24.54 ${ }^{\circ }$; 25.75 ${ }^{\circ }$; 26.91 ${ }^{\circ }$; 27.38 ${ }^{\circ }$; 27.99 ${ }^{\circ }$; 32.30 ${ }^{\circ }$; 33.08 ${ }^{\circ }$; 35.04 ${ }^{\circ }$; 37.60 ${ }^{\circ }$; 46.31 ${ }^{\circ }$; and 48.54 ${ }^{\circ }$ with miller index (102), (002), (112), (121), (012), (211), (202), (212), (113), (223), and (104). The synthesized Bi₂O₃ material is in the $\alpha$-Bi₂O₃ phase. These results indicate that the Bi₂O₃ material has been completely formed and has an excellent level of stability at low temperatures (Chen et al., 2011). There is a difference in intensity in the XRD pattern, but it does not significantly change the lattice structure. Based on the Debbye-Scherrer equation, the crystallite size of Bi₂O₃ with the addition of Fe from 0–9%, respectively, was 25.7 nm, 27.1 nm, 29.1 nm, 23.7 nm, 24.0 nm, and 21.7 nm. The addition of Fe resulted in differences in the size of the resulting crystals. The smaller the FWHM, the crystallinity is better .

Figure 3. X-ray diffraction patterns of Bi₂O₃/Fe 0–9%.

The resulting crystallite size is inversely proportional to the FWHM value (Table 1), while the intensity of each crystal plane influences the FWHM value, the higher the intensity, the smaller the FWHM value. The smaller the FWHM value, the better crystal quality because it shows that adjacent atoms are easier to adjust the direction and length of their bonds (Suryanarayana, 1998).

Table 1. 2 $\mathit{\theta }$, FWHM, and crystallite size of Bi₂O₃/Fe

Sample	2 $\theta$ (${ }^{\circ }$)	FWHM (rad)	${\mathit{D}}_s$ (nm)
0%	27.40	0.172	25.7
1%	27.38	0.168	27.1
3%	27.42	0.142	29.1
5%	27.77	0.232	23.7
7%	27.83	0.234	24.0
9%	27.78	0.273	21.7

Increasing the concentration of Fe from 5% to 9% causes the emergence of a new phase at 2θ = 41.79 ${ }^{\circ }$; 43.77 ${ }^{\circ }$; 45.62 ${ }^{\circ }$; 52.62 ${ }^{\circ }$; 54.26 ${ }^{\circ }$; 55.88 ${ }^{\circ };$ and 62.05 ${ }^{\circ }$ with miller index (323), (422), (341), (343), (424), (352), and (613) indicated to be the bismuth iron oxide (Bi₂₅FeO₄₀) phase according to JCPDS No. 46–0416 (Basith et al., 2018). The phase percentage between $\alpha$-Bi₂O₃ and Bi₂₅FeO₄₀ was 82.6%: 17.4%. It indicates that the 5–9% Fe doped still has a main phase of $\alpha$-Bi₂O₃, but there is a secondary phase of Bi₂₅FeO₄₀. The secondary phase is formed due to the addition of a larger Fe dopant concentration in Bi₂O₃, which causes the Bi and Fe atoms to undergo a reaction so that Fe should partially replace the position of the Bi atom, but due to the high concentration of Fe atom, the position of the Bi atom is completely replaced by Fe atom (Yuwita et al., 2019). The appropriate Fe dopant can increase the crystallite size. However, more than 3% Fe dopant causes the crystallite size to decrease due to the reaction between Fe and Bi atoms. The difference influences the reaction in the ionic radius of Fe³⁺ (0.69 Å) smaller than the ionic radius of Bi³⁺ (1.17 Å). The difference in atomic radius inhibits the Bi nucleation process because the increasing presence of Fe restrains it.

SEM characterization was carried out to determine the surface morphology of the sample. The characterization image can be seen in Figure 4(a-f). Pure Bi₂O₃ particles are indicated to have like a rod structure (Mukholit et al., 2019), but due to the high agglomeration rate, the rod particles accumulate in one area and form spheres (Figure 4(a)). The addition of 1% Fe causes the particle surface to become porous (Figure 4(b)). The addition of 3–9% Fe displayed a porous sphere and rod (Figures 4(c-f)). Particles with a porous spherical shape are indicated as Fe particles (McRae et al., 2019). The number of particles resembling a porous sphere increases with increasing Fe doping concentration. This is consistent with the results of the SEM-EDX mapping (Table 2). The addition of Fe concentration from 1% to 9% increased the %mass and %atoms Fe in the sample. The mapping results also show that the resulting sample contains Bi, O, and Fe elements. However, increasing the concentration of Fe doping resulted in a decrease in the particle size of Bi₂O₃ because the crystal Bi³⁺ ions were retained during the sintering process. When Fe³⁺ ions are inserted into the Bi₂O₃ crystal lattice, a certain degree of deformation will be introduced into the lattice due to the difference in ionic radius between Fe³⁺ ions (0.69 Å) and Bi³⁺ ions (1.17 Å), this is similar to the description of previous studies (Liang et al., 2014). Particle size was measured using Image J software. The average particle sizes of Bi₂O₃ 0–9% were 1.5 $\mu$ m, 1.2 $\mu$ m, 4.6 $\mu$ m, 3.6 $\mu$ m, 2.1 $\mu$ m, and 1.6 $\mu$ m. While the average particle sizes of Fe doped 1–9% were 1.5 $\mu$ m, 2.4 $\mu$ m, 2.8 $\mu$ m, 2.2 $\mu$ m, and 1.8 $\mu$ m. Particle size measurement through SEM image has a larger size than XRD pattern crystallite size measurement because the particles group together which causes agglomeration during synthesis (Hernowo & Nurhasanah, 2019). The structure of Fe, which resembles a porous sphere, can increase the photocatalytic activity of Bi₂O₃. One of the requirements for a good photocatalyst material is to have a porous surface to increase the surface area for photon absorption during photocatalysis. The large surface area results in more vacancies, thereby increasing the diffusion between AMX pollutant molecules and the photocatalyst material (Rissa et al., 2012; Sistesya & Sutanto, 2013).

Figure 4. SEM Image of Bi₂O₃/Fe: (a) 0%, (b) 1%, (c) 3%, (d) 5%, (e) 7%, (f) 9%.

Table 2. Result of SEM-EDX

Component	Element	0%	1%	3%	5%	7%	9%
Mass (%)	Bi	94.66	91.04	85.23	87.19	91.35	87.52
	O	5.34	8.65	13.99	11.85	7.57	11.08
	Fe	0	0.31	0.78	0.96	1.08	1.40
Atomic (%)	Bi	57.56	44.36	31.46	35.50	47.03	36.85
	O	42.44	55.08	67.46	63.03	50.89	60.94
	Fe	0	0.56	1.08	1.47	2.08	2.21

The results of the Surface Area Analyzer (SAA) to the Bi₂O₃/Fe samples showed the specific surface area of the Brunauer-Emmet-Teller (BET). The specific surface area is calculated with N₂ adsorption-desorption analysis that obtained successive results for Bi₂O₃/Fe 0–9% were 3.975 m²/g, 3.354 m²/g, 6.864 m²/g, 2.258 m²/g, 3.634 m²/g, and 3.069 m²/g. Bi₂O₃/Fe 3% has an optimal specific surface area so that it has the best potential in the AMX degradation process. The specific surface area of a large catalyst powder can increase the amount of AMX absorbed on the surface of the Bi₂O₃/Fe powder (Nurhasni et al., 2018).

The energy gap value (Figure Figure 5) is obtained by plotting $\alpha h{\nu }^{\frac{k}{2}}$ and $hv$ of the transmittance spectrum using the Tauc Plot method. Calculation of Energy gap (Eg) using Equation 3(3) $\alpha h\nu =A{\left(h\nu -E_g\right)}^{\frac{k}{2}}$(3) (Raizada et al., 2017).

Figure 5. The energy gap of Bi₂O₃/Fe: (a) 0%, (b) 1%, (c) 3%, (d) 5%, (e) 7%, f (9%).

(3) $\alpha h\nu =A{\left(h\nu -E_g\right)}^{\frac{k}{2}}$(3)

The index (k) is determined from the Bi₂O₃ semiconductor optical transition type, namely 4 for indirect transition (Liang et al., 2014) hv is the photon energy, A is the proportionality constant, and indicates the photon energy the absorption coefficient. The synthesized Bi₂O₃/Fe has an Eg of about 2.02–2.81 eV, presented in Table 2. The addition of a suitable Fe dopant can reduce the energy bandgap of the semiconductor. The minimum energy gap is obtained at the addition of 3% Fe, which is 2.02 eV. A small energy gap indicates that the energy band gap is getting narrower between the valence band and the conduction band. So that when excitation occurs due to the influence of photon energy from UV light irradiation, it will facilitate the production of electrons and holes because the energy needed for electrons to move from the valence band to the conduction band is getting smaller. The higher the production of electrons and holes can increase the photocatalytic activity.

The degradation of AMX determined the photocatalytic activity of Bi₂O₃/Fe. This photocatalytic activity shows a decrease in absorbance, efficiency, and degradation rate constant. Figure 6(a). showed that the absorbance value of AMX decreased with the length of irradiation time. The longer duration of irradiation resulted in more degrading species from the active Bi₂O₃/Fe photocatalyst so the greater the concentration of degraded AMX. The efficiency of the Bi₂O₃/Fe photocatalyst in degrading AMX is shown in Figure 6(b). The highest efficiency was obtained at 3% Fe doping, which was 76.00%.

Figure 6. (a) AMX absorbance decrease graph (b) Bi₂O₃/Fe degradation efficiency (c) Bi₂O₃/Fe degradation rate.

The rate degradation of Bi₂O₃/Fe concerning AMX according to the Langmuir–Hinshelwood (L–H) kinetic model, determined by the pseudo-first-order kinetic equation (Equation 4(4) $ln\frac{C_0}{C}=k_{app}t$(4) )

(4) $ln\frac{C_0}{C}=k_{app}t$(4)

$C_0$ is the initial AMX concentration (mg/l), C is the AMX concentration at a certain time (t) (mg/l), $k_{app}$ is the pseudo-first-order rate constant (min⁻¹) (Qin et al., 2014). The results can be seen in Figure 6(c). and in Table 3. The calculation results of the degradation rate explain that the 3% Bi₂O₃/Fe sample has the highest degradation rate constant of 0.0079 min⁻¹, while 9% Bi₂O₃/Fe has the lowest degradation rate constant of 0.0048 min⁻¹. The rate of degradation describes the speed of the material in degrading waste, the higher the value of the rate of degradation, the faster the material in degrading waste, so that the waste contained in the material is less.

Table 3. Efficiency, degradation rate, and energy gap of Bi₂O₃/Fe

Sample	Efficiency (%)	Degradation rate (min^−1)	Eg (eV)
0%	59.14	0.0051	2.43
1%	65.59	0.0059	2.37
3%	76.00	0.0079	2.02
5%	72.04	0.0071	2.30
7%	60.22	0.0049	2.71
9%	58.06	0.0048	2.81

The photocatalytic activity of Fe-doped Bi₂O₃ is higher than that of pure Bi₂O₃ due to the synergistic effect of Fe³⁺ ions and Bi₂O₃. The 3% Bi₂O₃/Fe material has the most significant degradation efficiency. These results indicate that adding Fe up to a concentration limit of 3% in Bi₂O₃ will increase the photocatalytic activity, while the addition of more than 3% will decrease photocatalytic activity. These results are consistent with previous research by Liang et al. (2014). Photocatalytic activity and degradation efficiency can be related to the crystallite size of Bi₂O₃/Fe (Nandiyanto et al., 2020), where the larger the crystallite size of a material, the greater the photocatalytic ability (Armaković et al., 2017; Sharma et al., 2014). In addition, the presence of Fe particles resembling a porous sphere on the surface of the Bi₂O₃ semiconductor as shown in the SEM image also affects the photocatalytic activity. The porous surface can increase the photocatalyst material’s surface area, and the presence of Fe transition metal can suppress the electron–hole recombination rate. This is also under the results of the BET analysis, a large specific surface area can increase photocatalytic activity. The value of the obtained Bi₂O₃/Fe energy gap also affects the photocatalytic activity. As presented in Table 3. The minimum Eg occurs at 3% Fe doping, following the results of the photocatalytic activity test, where the efficiency and optimal rate also occur at 3% Fe doping.

Chemically, the degradation of AMX by Bi₂O₃/Fe can be explained as shown in Figure Figure 7. When Bi₂O₃ is irradiated by UV light, the electrons in the photocatalyst material Bi₂O₃ which are in the valence band will be excited to the conduction band. This results in more electrons in the conduction band, while in the valence band there will be many holes formed due to electrons being excited to the conduction band. Electrons that do not recombine will be trapped on the surface of the semiconductor. Photogenerated electrons and holes combine with Fe³⁺ ions to form Fe⁴⁺ and Fe²⁺ ions (Equation 5(5) $B{i}_2{O}_3+hv\to e^{-}+h^{+}$(5) –8(8) $F{e}^{3+}+h_{vb}^{+}\to F{e}^{4+}$(8) ).

Figure 7. AMX degradation mechanism by Bi₂O₃/Fe.

(5) $B{i}_2{O}_3+hv\to e^{-}+h^{+}$(5)

(6) $O_2+B{i}_2{O}_3(e_{cb}^{-})+\to {\hspace{0.17em}}^{\bullet }{O}_2^{-}$(6)

(7) $F{e}^{3+}+e^{-}\to F{e}^{2+}$(7)

(8) $F{e}^{3+}+h_{vb}^{+}\to F{e}^{4+}$(8)

Fe²⁺ and Fe⁴⁺ ions are relatively unstable ions when compared to Fe³⁺ ions. Therefore, the trapped charge can be easily released from the Fe²⁺ ions or Fe⁴⁺ ions and then migrate to the surface to start the photocatalytic reaction with the following reaction:

(9) $F{e}^{2+}+O_2\to F{e}^{3+}+{\hspace{0.17em}}^{\bullet }{O}_2^{-}$(9)

(10) $F{e}^{4+}+O{H}^{-}\to F{e}^{3+}+{\hspace{0.17em}}^{\bullet }OH$(10)

Fe²⁺ ions can be oxidized to Fe³⁺ ions by transferring electrons to O₂ adsorbed on the surface of Bi₂O₃. Meanwhile, the adsorbed O₂ is reduced to superoxide anion radical (${\hspace{0.17em}}^{\bullet }{O}_2^{-}$) ; Equation 9(9) $F{e}^{2+}+O_2\to F{e}^{3+}+{\hspace{0.17em}}^{\bullet }{O}_2^{-}$(9) ), which functions as a reducing agent and further reduces the AMX solution. Likewise, Fe⁴⁺ ions are reduced to Fe³⁺ ions by losing electrons, while the surface hydroxyl group (OH⁻) becomes a hydroxyl radical (${\hspace{0.17em}}^{\bullet }OH$) Equation 10)(10) $F{e}^{4+}+O{H}^{-}\to F{e}^{3+}+{\hspace{0.17em}}^{\bullet }OH$(10) . In addition, the unrecombined holes will be trapped on the surface of the Bi₂O₃ semiconductor and react with H₂O and form hydroxyl radicals (${\hspace{0.17em}}^{\bullet }OH$). The resulting hydroxyl radical is a potent reducing agent to reduce AMX solution (Saravanan et al., 2013).

The degradation process occurs through direct charge transfer consisting of photoinduction carriers or reactive oxygen species (ROS; Guo et al., 2011). The ROS that contributes to the photocatalytic reaction of Bi₂O₃/Fe as a robust oxidizing and reducing species to degrade AMX are superoxide anion radicals (${\hspace{0.17em}}^{\bullet }{O}_2^{-}$) and hydroxyl radicals (${\hspace{0.17em}}^{\bullet }OH$) which will form CO₂, H₂O, and other inorganic components (Equation 11(11) ${\hspace{0.17em}}^{\bullet }{O}_2^{-}+H^{+}\to {\hspace{0.17em}}^{\bullet }OOH$(11) –15(15) $h^{+}+AMX\to C{O}_2+H_{2}O+\mathrm{i}\mathrm{n}\mathrm{o}\mathrm{r}\mathrm{g}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{c}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{d}\mathrm{s}$(15) ). These chemical reactions will bind to AMX pollutants around the Bi₂O₃/Fe material to degrade AMX pollutants. These radicals will be formed continuously as long as the Bi₂O₃/Fe material is exposed to UV light.

(11) ${\hspace{0.17em}}^{\bullet }{O}_2^{-}+H^{+}\to {\hspace{0.17em}}^{\bullet }OOH$(11)

(12) ${\hspace{0.17em}}^{\bullet }{O}_2^{-}+H^{+}+{\hspace{0.17em}}^{\bullet }OOH\to H_2{O}_2+O_2$(12)

(13) $H_2{O}_2+O_2^{-}\to OH+O{H}^{-}+O_2$(13)

(14) ${\hspace{0.17em}}^{\bullet }AM{X}^{+}+({\hspace{0.17em}}^{\bullet }OH,{\hspace{0.17em}}^{\bullet }{O}_2^{-}+O_2)\to C{O}_2+H_{2}O+\mathrm{i}\mathrm{n}\mathrm{o}\mathrm{r}\mathrm{g}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{c}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{d}\mathrm{s}$(14)

(15) $h^{+}+AMX\to C{O}_2+H_{2}O+\mathrm{i}\mathrm{n}\mathrm{o}\mathrm{r}\mathrm{g}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{c}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{d}\mathrm{s}$(15)

In the presence of Fe transition metal doping, Fe³⁺ ions are responsible for the electron-hole recombination rate and support the increase in photocatalytic activity. Excess Fe³⁺ ions is entering the cluster formation. This cluster can resist AMX photodegradation by masking the active site from the Bi₂O₃ surface. Fe³⁺ ions will act as photo-generated between holes and electron transfer, the rate of electron-hole recombination during irradiation can be suppressed by increasing the number of trapped electrons to increase the lifetime of electrons and holes. This decrease in the recombination rate increases the photocatalytic activity of Bi₂O₃/Fe.

The Bi₂O₃/Fe material produced is in powder form, so that in its application, it can only be used once to degrade AMX medical waste, the possibility of recycling can be carried out further by coating the material on a glass substrate to produce Bi₂O₃/Fe thin-film material. Materials in the form of thin films can be applied repeatedly in the degradation process (Sutanto et al., 2021).

To carry out degradation under solar irradiation, it is necessary to modify the energy bandgap of the Bi₂O₃/Fe material which corresponds to the photon energy of visible light, which is smaller than 3.1 eV. In this study, researchers have modified the energy band gap of Bi₂O₃/Fe to 2.81–2.02 eV, so that in principle it is possible for electron transfer and photocatalytic reactions to occur even under sunlight.

4 Conclusion

The synthesis of Bi₂O₃/Fe has been successfully carried out using the microwave-assisted precipitation method. XRD characterization results showed that the phase formed was α-Bi₂O₃. The morphology of Bi₂O₃ is porous rods and spheres indicated as Bi₂O₃ and Fe particles. Bi₂O₃/Fe 3% has the largest crystallite size (29.1 nm), the lowest energy gap (2.02 eV), the most optimal specific surface area (6.864 m²/g), the highest degradation rate (0.0079 min⁻¹), and the most optimal degradation efficiency (76.34%). The addition of Fe with a concentration of up to 3% in Bi₂O₃ can increase the photocatalytic ability, but a concentration of more than 3% will decrease the photocatalytic ability.

Acknowledgements

This research was funded by Kementerian Riset dan Teknologi/BRIN Republik Indonesia for Penelitian Dasar Unggulan Perguruan Tinggi (PDUPT) scheme with contract number: 187-24/UN7.6.1/PP/2021.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Ministry of Research, Technology, and Higher Education – Indonesian [187-24/UN7.6.1/PP/2021].

Notes on contributors

Heri Sutanto

Heri Sutanto is a Full Professor in the Department of Physics, Diponegoro University. Heri Sutanto also leads the Smart Materials Research Center (SMARC), Center of Research and Services-Diponegoro University. He has been doing research in materials science, especially in the development of nanomaterials, semiconductors, thin film, photocatalyst, and radioprotector medical materials, and also focuses on energy-environmental application. He has many publications in scientific journals and recipient of numerous awards both national and international. Bella Aprimanti Utami is a magister student at the Department of Physics, Diponegoro University. This reported research is part of a roadmap for the development of environmentally friendly advanced materials for the degradation of medical waste by the SMARC laboratory.