At the present time, the excess of residual liquids has become in a critical problem. There are a lot of pollutants in the water, among which are colorants and chemical substances. Industrial activities are significant contributors to the wastewater pollution of all the water sources in the world, different industries, among which is textile as one of the largest generators of wastewater due to the dyeing and finishing process, where azo dyes are used . There are a lot of attractive approaches that has been used as treatment for this wastewater. Heterogeneous photocatalysis is an advanced oxidation process widely used to the degradation of organic pollutants in water. It used semiconductors as catalyst for this process, normally. TiO2 powder is employed for this purpose, however, different and economical materials as Bi2O3 have been studied to be used as substitute or complementary system .
On the other hand, the most used catalyst is titanium dioxide, TiO2, which has proved that is highly efficient in degradation process of organic materials due to photocatalytic and photoconductor properties. This material has a band gap of 3.2 eV, which means that all UV radiation during a wavelength of 387 nm or less will have enough energy to turn on the catalyst . It also has interesting properties such as transparency in visible light, refractive index, and low absorption coefficient . Titanium dioxide has three different polymorphic structures: rutile (tetragonal), anatase (tetragonal) and brookite (orthorhombic). Also, it is a semi-conductor, usually during its anatase and rutile phase .
There have been many researches to know bismuth oxide’s physicochemical properties. As a result, there is known that bismuth oxide has different structures which can be obtained with micro and nanometric structures. Besides this polymorphic characteristic, bismuth oxide is characterized for being a well ionic conductor because its band gap is between 2.7 - 2.8 eV and, it has a good photoconductivity .
Bismuth (III) oxide is one of the simplest oxides and it has the advantage of being economical and environmentally friendly. Nowadays, this oxide is used in different areas such as medicine and engineering, and also is used in cosmetics because of its low toxicity and, in the last few years, 43% of the bismuth production has been used as a replacement of lead .
According to several researches, this oxide has six polymorphic structures, yet, four of them are the most common: α, β, γ, δ, which two of them are stable; monocyclic phase α works at low temperatures and, cubic phase δ works at high temperatures, approximately at 1003.15 K; this phase is known because of its high ionic conductivity as oxide ions can move from side to side along bismuth arrangement. The other two intermediate meta-stables phases: tetragonal phase β occurs at 923.15 K and, the γ phase has a body centered cubic structure which is formed at 913.15 K .
In terms of this research, Mexico has maintained its second place in bismuth’s world production with a maximum quantity of 1000 ton in 2010 , so bismuth oxide can replace titanium dioxide as catalyst.
For this research, α stable structure of bismuth oxide carbonate was used. It has parallel layers of bismuth atoms that are separated by layers of oxide ions. This allows the structure to have better superficial area. Also, due to low fusion point of bismuth, most of the methods that are used to synthesize different structures do not apply to bismuth. Bismuth oxide carbonate was synthesized via solvothermal; however, to avoid long reaction periods, the experiment was microwave-assisted, which produces nanoparticles with low size dispersion, although is hard to have control on morphology. This technique has been used during the last few years as an alternative method to synthesize materials at a nanometric scale because it is a fast and effective method that allows to get more quantity of product in a short period of time . The synthesized material was tested as catalyst of heterogeneous photocatalysis in the reaction of degradation of methylene blue, as azo dye used in many textile industries.
2. Experimental Procedure
2.1. Preparation of the Bi2O2CO3 Catalyst
BiNaO3 (Sigma Aldrich) was dissolved in ethylene glycol (Reasol) to produce a 0.03M solution and maintained continuous stirring at 700 rpm for 15 min, then, the resulting solution was transferred into a digestion vessel (Parr mod. 4781). After that, the vessel was introduced into a microwave oven (Amana Commercial Mod. HDC12A2), the solution was irradiated at 1200 Watts for 10 seconds. Finally, the precipitates were centrifuged and washed several times using distilled water and isopropyl alcohol intercalary. The resulting precipitates were dried for 1 h in an oven (Neytech Qex mod. V1.2) at 100˚C for 1 hour.
2.2. Characterization Details
The surface morphology of the samples of commercial Bi2O3, Bi2O2CO3 synthesized and commercial TiO2 was characterized by scanning electron microscopy (SEM) in a JEOL microscope model JSM-7800F, with a resolution of 0.8 nm at 15 kV of acceleration in secondary electrons mode. A Rigaku diffractometer, model Miniflex 600 with X ray tube with Cu Ka (l = 1.54 A˚) radiation and lineal focus was used for XRD, with 40 kV y 15 mA. Symmetric measurements from 3 to 70 degrees with step of 0.01 and a speed or 3 degrees/minute. Nitrogen adsorption and desorption isotherms were measured at 77 K using Quantachrome Autosorb 1. Prior to the measurements, the Bi2O3 0.03 M and TiO2 samples were outgassed at 100˚C for 25 h. and the commercial Bi2O3 sample was outgassed as the same temperature like the previous samples for 43 h. The specific surface area and the pore size distribution (average pore diameter and mean pore volume) were measured from the adsorption isotherm using the Brunauer-Emmett-Teller method and Barrett-Joyner-Halenda method, respectively.
2.3. Degradation of Methylene Blue
To verify the degradation of methylene blue (C16H18ClN3S), it was prepared a 25-ppm solution (Sigma Aldrich). First a photolysis was made using 50 ml of the 25-ppm solution that was transferred to a quartz reactor. To make this experiment it was used a closed equipment that contained 8 UV lamps (50 W), and the solution was stirred at 600 rpm. During irradiation, 5 ml of the solution was removed every 10 min for 2 h and went back inside the solution after being analyzed using a Perkin Elmer spectrophotometer 365 in a range of 400 - 700 nm to measure the absorbance of each sample. The commercial Bi2O3, synthesized Bi2O2CO3 and TiO2 catalyst were also studied under the same conditions.
3. Results and Discussion
3.1. DRX and SEM Analysis
The morphology of the samples was observed by SEM. Figure 1(a) shows the surface of the synthesized Bi2O3 sample composed by agglomerated foils of 5 µm maximum that might have very good results in photocatalytic activity because of the porosity found on the structure. These foils started to grow during the first 10 seconds of irradiation. The next two images are from the commercial Bi2O3 (Figure 1(b)) and TiO2 (Figure 1(c)) respectively, these structures show difference between morphology and how it affects the degradation of methylene blue because the images show that TiO2 particles are sphere shaped with an average diameter between 100 to 200 nm, while Bi2O3 shows compact agglomerate structures with a diameter of 10 µm.
According to diffraction pattern, Figure 1(a) corresponds to a body centered tetragonal crystalline structure with unit cell parameters a = b = 3.865 and c = 13.675 and angles of α. β, Υ de 90˚, this information matches accurately to the crystallographic PDF chart 41-1488, of the bismuth oxide carbonate (Bi2O2CO3). Figure 1(b) shows the diffraction pattern of the alpha bismuth oxide (Bi2O3), where the corresponding PDF chart is 71-0465 and demonstrated the presence of a monoclinic system with space group P-21/c(14) and lattice parameters of a = 5.849, b = 8.164 and c = 7.510 and the angle β = 112.97. Finally, Figure 1(c) show the XRD of commercial TiO2 anatase phase agree with the PDF chart 89-4921 where tetragonal system is reported with lattice parameters of a = b = 3.77 and c = 9.501, angles of 90˚ and space group I41/ad(141).
3.2. Nitrogen Adsorption - Desorption Measurements
The nitrogen adsorption-desorption isotherms of TiO2 and Bi2O3 structures are shown in Figure 2. The TiO2 and synthesized Bi2O3 are a type II isotherm curves. It is observed that the synthesized Bi2O3 adsorbed better the N2 due to a bigger surface area and pore radius. The commercial Bi2O3 is a type III isotherm due to the small surface area and pore volume in comparison to the other isotherms.
The corresponding average pore diameter and total pore volume are calculated from the BJH pore size distribution curves and the values are given in Table 1, it is clear that the molecules synthesized of Bi2O2CO3 exhibit a superior surface area and also a higher pore diameter and pore volume than TiO2 particles. The commercial Bi2O3 surface area is 5.64 times smaller than the synthesized sample and it is because a precursor solution with low concentration was used for the synthesis and also the small period of time on the microwave affected positively the porosity of the structure. The specific surface area for the Bi2O2CO3, Bi2O3 and TiO2 materials is also listed in Table 1.
3.3. Optical Properties
The band gap energy was measured using a Perkin Elmer spectrophotometer 365 with integration sphere at wavelength of 500 to 300 nm in Absorbance mode. The Kubelka-Munk method was used to determine the band gap energy. According to the Kubelka-Munk method the Equation (1) was be used:
Table 1. Surface area, total pore volume, and average pore diameter of commercial Bi2O3, anatase TiO2 and synthesized Bi2O2CO3.
Figure 1. XDR and SEM micrograph of (a) Synthesized Bi2O2CO3, (b) commercial Bi2O3, (c) commercial TiO2.
Figure 2. N2 adsorption-desorption isotherms, (a) Bi2O2CO3, (b) commercial Bi2O3, (c) commercial TiO2, (d) isotherm comparison.
where α, υ, A, Eg are the absorption coefficient, light frequency, a constant and the band gap energy, the n is determined by the type of optical transition in the semiconductor (i.e. n = 2 for the direct transition and n = 1/2 for the indirect transition). Taking in consideration that the samples are an indirect semiconductor, the band gap energy is determined from a plot of vs. Band gap energy, obtaining the results shown in Table 2.
As it can be observed, the value of Bi2O2CO3 band gap is inside the range of the theoretical values which supports the theory that it is actual Bi2O2CO3 and not another compound, and because of its higher value, it needs more energy than anatase, that is one of the reasons that degradation percentage is slower than anatase.
3.4. Kinetics of Degradation of Methylene Blue
The degradation of methylene blue is until the final stage of the process shown in Figure 3, give as result the formation of CO2 and water, removing the dye in the original solution. In this work, we follow the reaction only with the dye concentration respect the time.
Figure 3. Degradation process of the methylene blue .
Table 2. Band gap data obtained from the Kubelka-Munk method and the theorical ones.
The experimental section of the application was carry out following the next procedure: first a calibration curve was obtained, in order to know the concentration of dye in the solution, a photolysis to know the degradation caused by the lamps and finally, the three catalyst were proved at the same conditions.
The calibration curve of methylene blue was obtained to calculate the concentration of the solutions in relation to time. It is clearly observed that the adjusted R-squared value is equal to 0.99756, with an adjusted equation y = 0.1103x − 0.01103. This data allows to obtain the concentration of each sample of methylene blue solutions that were taken during the degradation experiments.
To obtain the data for Table 3 some equations were used. Langmuir-Hinshelwood was employed to obtain the first approximation, considering a first order kinetics, as is used in organic dyes in low concentrations. The different concentrations of each solution were calculated using the adjusted equation of the calibration curve (Equation (2)).
The equation above needs the Absorbance in different time (Abs), and the result is the concentration of methylene blue in the solution (CMBS). Then, the percentage of methylene blue in solution was calculated in order to obtain the degradation efficiency, the Equation (3) was used
CO is the initial concentration of each experiment and finally, the degradation efficiency of each catalyst was calculated by the Equation (4):
where DMB is the degradation efficiency of catalysts and MBs is the percentage of methylene blue in solution. The table below shows the degraded methylene blue percentage in relation to time.
Figure 4 shows the comparison of the different experiments to show the degradation percentage of the methylene blue along the 120 min. TiO2 degrades 85.64% of the solution while Bi2O2CO3 shows a degradation percentage of 68.25, showing that the morphology affects positively on the photocatalytic activity. Morphology plays an important role in photocatalytic activity because commercial Bi2O3 is formed by agglomerated compacted structure that reduces its surface area. A serial of factors influence the photocatalytic reaction with Bi2O3, among which are: due to its hybridation (Bi 6s and O 2p orbital), it is a strong oxidizer and can successfully degrade organic compounds and favors the mobility of photoholes in the valence band .
Table 3. Degraded methylene blue percentage.
Figure 4. Degradation curves of methylene blue.
According to Levenspiel, the equation used to get the parameters come from the reaction rate equation in terms of the concentration of the reactant
where n means the order of the reaction, k is the rate constant and the CA is the concentration of the reactant, which in this case is methylene blue.
It is well known that the concentration of the reactants in time t can be explained by the Equation (6)
CA0 is the initial concentration of the catalyst and XA is the degree of conversion of the reactant. Then, the substitution of Equation (6) in Equation (5) is next
Knowing the time of the experiment, the change of concentration in time t due to absorbance measurement and the initial concentration of each experiment, the equation to know the rate constant is shown below as Equation (8).
Table 4 shows the results of the calculus made to get the approximately kinetic constant of each catalyst and to understand their behavior during the photocatalysis. Even that they are first order kinetics, the speed constant of Bi2O2CO3 shows a bigger constant than TiO2 Anatase because during the first 20 minutes of UV radiation, Bi2O2CO3 degrades better than TiO2. The same behavior is shown with Commercial Bi2O3. For the experiment anatase TiO2 was used because its properties are well known, the superficial area has a value of 10 - 17 m2/g  . TiO2 anatase band gap experimental value is 3.0 eV which shows that needs more energy to get excited and to degrade in a higher way.
Table 4. Kinetic parameters of the catalysts used.
Anatase TiO2 clearly has the highest degradation percentage of all samples. However, the Bi2O3 structure that was synthesized by solvothermal method using microwave irradiation shows a variation in the formula indicating traces of CO3 but also shows an acceptable degradation percentage according to TiO2 results, making Bi2O2CO3 a possible substitution of TiO2. It needs to be mentioned that BET area shows that Bi2O2CO3 is clearly superior than Anatase TiO2 including pore radius. This will indicate in the future that with the proper conditions, it can degrade better than TiO2. Also, the morphology highly affects degradation process: if the structure possesses high porosity, it will have a high degradation percentage. In terms of band gap, the experimental value of Bi2O2CO3 is inside of theoretical values, but the amount of energy needed in the degradation experiment is higher than anatase; that is one of the principal reasons why anatase has a higher degradation percentage, however, Bi2O2CO3 presents acceptable results, especially in the first minutes of reaction. It is important to consider that Mexico has the second place in bismuth oxide production worldwide, and this allows to reduce costs and to get the catalyst inside the country. Wastewater problem could be decreased considerably using a system like the presented in this research.
Authors are thankful to IPN for the support to SIP projects: 20196017, 20190616 and 20190251.
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