Heterogeneous catalytic reactions involve a series of physical transport steps that can influence the overall reaction rate. In photocatalytic systems, these steps occur sequentially, as described by Vazzoler (2019) :

1) Initial movement of reactants or microorganisms through a boundary layer adjacent to the catalyst surface.

2) Internal transport within the catalyst pores, allowing the reactants to reach the active sites (intraparticle diffusion).

3) Interaction of reactants with the catalyst surface, leading to adsorption.

4) Chemical conversion at the active sites, followed by the release of products (desorption).

5) Transport of products within the pores, moving toward the catalyst’s external surface.

6) Dispersion of products through the external boundary layer, enabling their release into the surrounding fluid.

The main chemical transformations associated with photocatalytic decomposition and disinfection occur during steps 3 and 4, where the catalyst’s surface area plays a crucial role. However, the other steps can also affect the process’s overall efficiency. Typically, the slowest step serves as the limiting factor, determining the overall reaction behavior.

The kinetic study of these systems aims to determine the rate of chemical reactions and identify the factors influencing them. For this purpose, pseudo-first-order equations (Equation 2), pseudo-second-order equations (Equation 3), and intraparticle transport equations (Equation 4) are commonly applied ( Ray, Dhakal, & Lee, 2017 ).

The TiO₂-Dt semiconductor synthesis allows its use in the tests with a magnetic stirrer using a beaker containing distilled water, glacial acetic acid, titanium isopropoxide, and natural diatomite. After adding the reagents and the distilled water, complete the volume until 400 ml with the final suspension mixed for 2 to 3 hours. After decantation and filtration, the solid formed by the sol-gel process was placed in an oven heated to an average temperature of 373K for 24 hours to obtain TiO₂-Dt microstructures. The prepared material suffers a drying step, storage, and use in solar photodecomposition.

The synthesis process of microstructured TiO₂-Dt was based on the hydrolysis of titanium isopropoxide, using natural diatomite as the structural template. The catalyst preparation used the sol-gel method. In the experiments described, TiO₂-Dt samples prepared with 8.0 g and 9.0 g of natural diatomite. With these proportions of diatomite addition of 5 mL of glacial acetic acid and 10 mL of titanium isopropoxide, the final volume was 400 mL adjusted with distilled water. The final suspension was stirred for 2 hours, and after 1 hour of decantation, the solid formed was placed in an oven heated to 373K for 24 hours to obtain microstructures shaped with TiO₂-Dt. The storage of the obtained material follows the drying process ( Figure 3 ).

The tests carried out in the Center for Environmental Chemistry—CEQMA laboratory at IPEN. The parameters of the photodecomposition process studied

were the mass proportion 9.0 g of TiO₂-Dt, the synthetic solutions of the water and BTEX % prepared in the laboratory, stirring time and incidence of artificial solar radiation, pH = 5.0 values, lux 380 and temperature 295 K - 298 K. The photocatalyst column filled with TiO₂-DT had the percolation process lasting for two hours. The control of the photodecomposition process applied by the collected aliquots after percolation and the aliquot analysis allows the determination of BTEX concentrations. The column preparation was with a fixed bed of TiO2-Dt, and the percolation process enabled the collection of aliquots at time flow intervals of 0, 30, 60, 90, and 120 minutes for BTEX photodecomposition ( Figure 4 ).

Preparing the BTEX synthetic solutions uses different volumes of gasoline purchased at the gas station commonly sold to the public. All the glassware used belongs to the CEQMA laboratory, and the acquisition of the titanium isopropoxide, acetic acid, and other reagents used were with a high level of purity and analytical grade.

Two LED lamps installed in the solar chamber simulated solar radiation, allowing solar photodecomposition to occur under controlled conditions. The BTEX solutions were prepared and diluted in different concentrations (BTEX and distilled water) at values equivalent to those found in the literature for contaminated waters.

The light chamber, designed by the author and built in the CEQMA laboratories, has the following dimensions: height of 45 cm, width of 55 cm, and depth of 55 cm. Fluorescent LED lamps were used as a source of artificial sunlight and located approximately 10 cm from the solution. The lux meter inside the light chamber measured the illuminance during the experiments. The lux measurements ranged from 380 to 390 lux.

After collecting the aliquots at different times of percolation and irradiation, the benzene content was controlled using the UV–visible spectrophotometer. Figure 5 illustrates the spectrophotometer connected to the computer for readings and allows observing the photodecomposition process.

The ordinary gasoline purchased in most gas stations has the most common toxic substance, the BTEX mixture. This aromatic mixture has a preponderance of the benzene content, with about 70% of the total. The analytical method to measure and control the BTEX water contamination is the use of Gas Chromatography with Mass detector (CG-MS).

The use of UV-VIS spectrophotometry helps to control the photodecomposition process by determining the benzene concentration in each collected aliquot at the characteristic wavelength of λ268 nanometers (nm).

1—BTEX solution with TiO₂-Dt;

2—Artificial solar radiation chamber;

3—Aliquot collections;

4—Analysis with UV-visible spectrophotometer.

The spectrophotometry principle is that each compound absorbs or transmits light at specific wavelengths. This relationship allows determining a compound’s concentration based on the amount of light absorbed or transmitted.

In essence, the technique leverages the interaction of light with matter to measure concentration. Every chemical compound absorbs, transmits, or reflects light within a particular wavelength range. Spectrophotometry quantifies the concentration of compounds in65 a solution by measuring the extent to which they absorb or transmit light. The amount of light absorbed or transmitted is directly proportional to the concentration of the substance in the solution ( Dias et al., 2016 ).

In this study, spectrometry was used to measure and control the photodecomposition process by measuring the absorbance of the benzene compound.

For the BTEX analysis using the equipment Shimadzu 17A gas chromatograph Kyoto -Japan allows the quantification of the organic compound content. The photoionization equipment and flame ionization detectors, with a split/splitless injector. A DB-624 megabore fused silica column (6% cyanopropylphenyl-94% dimethylpolysiloxane; 75 m × 0.53 mm ID, 3 μm), supplied by J&W Scientific (Folsom, CA, USA), was used. The carrier gas, helium, with a purity of 99.99%, was maintained at a flow rate of 10 mL/min.

During the process, the column temperature program was carefully controlled: initially, the temperature was set to 303 K for 1 minute, followed by an increase to 372 K at a rate of 278 K/min. Subsequently, a direct rise to 493 K at a 281 K/min rate occurred, maintaining at 493 K for 5 minutes. The injection port and detector temperatures were set at 453 K and 493 K, respectively. The PID system was operated with a 10 eV lamp and an intensity of 1 mA, as described by Dórea et al. (2006) .

Additionally, the GC-MS chromatography technique enabled the identification of the organic composition of the analyzed samples. This approach provided a detailed characterization of the compounds separated in different retention times; afterward, the sample has the content confirmation by the fragmentogram obtained in the Mass detector.

The maximum removal efficiency was achieved using 0.5 g and 0.8 g of TiO₂-Dt with dilution concentrations of 60% and 40% BTEX, respectively. Increasing the mass beyond this point decreases process efficiency as the system reaches saturation equilibrium. Table 2 presents the results.

By determining the optimal semiconductor mass for maximum efficiency, other process parameters could be optimized, and BTEX removal evaluated more effectively.

Source: Author.

The synthesis of titanium dioxide with diatomite was carried out in the laboratory under controlled conditions. Later, the synthetic material was subjected to exploratory photodecomposition tests and physical-chemical characterization using SEM and thermogravimetry analytical methods.

The Scanning Electron Microscope contains a source that generates an electron beam continuously fired at the sample during the test. The beam scans the sample’s surface and generates high-definition images, as in Figure 6(a) and Figure 6(b) .

The images generated by the SEM allow for different types of analyses on polymers, both structural and chemical.

Morphological information can be obtained in relation to structural analyses, such as the orientation and quality of the reinforcing fiber interface, the interface between the matrix and dispersed phase of immiscible blends, and the presence of impurities, bubbles, cracks, and irregular surfaces.

Thermogravimetric analysis ( Figure 7 ) verifies a substance’s thermal degradation, thus determining its thermal stability and best drying temperature.

The heating ramp varied between 273 and 1123 K, observing the variability of the semiconductor mass. From 873 K, there was a loss of 93.3% of the mass value compared to the initial value of 7.1 mg, and this is due to the use of diatomite with greater volatility as a structuring agent.

TiO₂-Dt dried in an oven at 373 K, and the material did not undergo calcination. Therefore, water and volatile compounds formed during the reaction evaporated, leaving a diatomite residue contributing to its synergistic effect during photodecomposition.

The x-ray diffractometry analysis confirmed the crystalline structure of TiO₂-Dt prepared with 0,05 g of diatomite used in the experiments, as the TiO₂ anatase. The diffraction pattern of the TiO₂-Dt material displayed diffraction peaks at 2θ = 25.3, 37.9, 47.9, 54.1, 62.6, 68.9, 69.7, 75.0, and 82.5 ( Figure 8 ).

The analyzed TiO₂-Dt sample indicated higher TiO₂ crystallinity due to the small amount of Dt added during the material’s synthesis.

The infrared analysis of the synthesized photocatalysts shows that the characteristic silica peaks remain unchanged before and after the photodecomposition process ( Figure 9 ).

Conversely, the peaks near 3500 cm⁻¹ are significantly increased in the saturated photocatalyst samples ( Figure 10 ).

Nevertheless, the structural peaks of the Si-Ti bond remain the same.

As shown in Table 3 , the silica composition represents the photocatalyst’s microstructural stability, the titanium oxide composition indicates the semiconductor’s efficiency in photocatalysis, and the loss on Ignition reflects the biochar’s thermal instability with potential structural changes after heating.

0.01%—XRF quantification limit; nd—not detected; Source: Author.

The adsorption tests were controlled by measuring benzene concentrations by spectrophotometric determination based on a calibration curve prepared with standard solutions. The concentration measurements of the complexes formed carried out with the aid of a UV-visible spectrophotometer device.

An Experimental Design matrix used the process parameters: the mass ratio between the initial concentration of benzene solutions and the mass of diatomite doped with TiO₂, the system’s pH values, and the temperature of the adsorption process.

The results allowed the construction of the benzene concentration variation curves in solutions at different percolation time intervals. Determining the best photodecomposition conditions, the system’s equilibrium concentration, saturation times, and the influence of pH and temperature.

Some photodecomposition processes were accomplished by changing the initial concentrations of BTEX and studying the effect of adding different masses of the semiconductor (TiO₂ Diatomite) at different intervals for 2 hours.

The tests with 60% BTEX and 40% H₂O with a mass of 1.0 g of diatomite TiO₂ presented satisfactory results, as shown in Table 4 .

Source: Autor.

The results indicate that moderate amounts of catalyst, such as 0.5 g to 1.5 g, are more effective for BTEX removal, while larger amounts may decrease efficiency of the photodecomposition reaction.

The experimental results showed better correspondence with Boltzmann sigmoidal curve, Figure 11 . The calculation of the different mass of TiO₂-Dt, and BTEX percentages under laboratory conditions with artificial light for the photocatalytic decomposition process. The equation corresponds with Qui ² = 12.00 and R² = 0.824.

Figure 12 shows the result of the Thomas model calculation.

The Thomas calculation, used to model the performance of contaminant photodecomposition, evaluates the BTEX removal capacity of catalytic materials.

In the tests with TiO₂-Dt and the initial 40% BTEX, the Thomas calculation revealed efficient photodecomposition, especially with 1.2 g of TiO₂-Dt, which demonstrated an increase in adsorption capacity due to the higher catalyst mass, providing effective removal, although there was no proportional increase in performance with higher masses.

In summary, the Thomas calculations showed that BTEX removal efficiency was mainly influenced by the amount of TiO₂-Dt and the BTEX percentage. The 60% BTEX demonstrated better performance with TiO₂-Dt, particularly when higher masses of TiO₂-Dt were used. The R2 = 0.952 indicate high correspondence with Thomas model.

The Yoon-Nelson calculation indicated efficient adsorption performance, with short breakthrough times and efficient removal capacity. Figure 13 shows the result of the Yoon-Nelson model calculation. The R² values of 0.869 indicate some correspondence with the model; more experimental data’s will improve the R² values.

Table 5 presents the comparison of the Thomas (K_Th) and Yoon-Nelson (K_YN) constants. The literature values and those obtained in this study, allows the visualization of the contaminant removal rates across different studies.

Source: Author.

Comparing the values obtained in this study with those presented in the literature, it is observed that they are of the same order of magnitude, although with a wider range of variation. This indicates that the parameters of the photodecomposition process are efficient, can be used and can still be improved and stabilized.

GC-MS chromatography analyses of synthetic water contaminated with initial 60% of BTEX identify the characteristics BTEX peaks in the beginning of the photodecomposition process (A) and after 2 h (B). Figure 14 shows the results with the retention time characteristics of benzene, toluene, ethylbenzene, and xylene compounds, after 2h there was no detectable contamination of BTEX, or there was about 100% of removal.

The optimization of the solar photocatalytic decomposition process offers a promising alternative for treating groundwater contaminated with BTEX. The results showed that TiO₂ microstructured with diatomite could remove up to 100% of BTEX from contaminated water. Additionally, the process allows for the recovery of leaked fuel from gasoline tanks by simply organic phase separation. The photocatalytic decomposition tests indicated that solar radiation is essential for the complete degradation of BTEX. The solar photocatalysis is crucial for the total removal of contaminants.

The TiO₂-Dt and solar photocatalysis proved an effective strategy for contaminated water remediation, offering a sustainable and accessible solution. The thermogravimetric analysis confirmed the stability of the photocatalyst, highlighting its importance for process efficiency.

The developed system is applicable in both open and closed areas, and field tests using natural solar radiation prove its effectiveness. Considering an open area the Solar energy can also power the system, making the photocatalyst system autonomous and cost-effective. Based on such promising results, there is a suggestion to implement a pilot system at a gas station contaminated site to evaluate the results on-site.

Optimizing the photodisinfection process can provide a new alternative also to the disinfection methods used in the country.

The results demonstrated that up to 100% of BTEX can be removed from BTEX contaminated water using TiO₂ microstructured with diatomite. In the process under study, it is still possible to recover the fuel leaked from the tanks and reuse it after organic phase separation.