Solar Heterogenous Photocatalytic Degradation of Methylthionine Chloride on a Flat Plate Reactor: Effect of pH and H₂O₂ Addition

Abstract

Methylthionine chloride (MTC) is a compound with several applications both in the clinical and medical industries. Nevertheless, such compounds can become an environmental problem, as they are not properly treated by wastewater treatment plants. This objective of this work was to study MTC degradation in a flat plate reactor through solar photolysis and heterogeneous photocatalysis processes with TiO₂ as a catalyst. In addition to the processes, three pH (3.5, 6.5, and 9) and the effect of H₂O₂ addition (no dose, 0.5, and 1 mM/L) were tested. The results show that acidic pH is the most appropriate for MTC degradation, which ranged between 56% and 68.7% for photolysis and between 76% and 86.7% in photocatalysis. The H₂O₂ addition resulted in lower degradation in all cases, leading the authors to conclude that the presence of peroxide actually hinders degradation in solar photolysis and photocatalysis processes. Statistical analysis showed that the constant rate reactions calculated for every process, under the same conditions of pH and H₂O₂ addition, are significantly different from one another, and the three factors considered for experimental design (process, pH, and H₂O₂) have a statistically significant effect on MTC degradation. The collector area per order confirmed higher efficiency for photocatalysis when compared to photolysis processes.

1. Introduction

Methylthionine chloride (MTC) is a cationic heterocyclic aromatic chemical compound with a molecular formula of C₁₆H₁₈ClN₃S (Figure 1). MTC is a green shiny powder that is very soluble in water and confers a blue color to the solution [1,2,3], which is usually known as methylene blue and is signally used in different industrial sectors [4], such as textile [5], chemical, and clinical, since it is considered a safe and effective substance [6,7,8].

MTC has been widely used in the clinical field for photodynamic therapy and as an indicator of breast cancer [9,10,11,12], for the treatment of urinary tract infections [13,14], or even malaria [15,16]. In textiles and painting industries, the MTC is used as a dye for painting different fabrics such as cotton, silk, wool, and leather [17,18]. MTC is also frequently used in aquaculture to cure diseases, disinfect, and transport fish [19,20]. At the laboratory level, MTC is used for the initial microscopic examination of macroscopically negative blood cultures [21].

MTC is a synthetic substance, with a chemical structure difficult to break, making it an extremely hard compound to remove by traditional mechanical or biological methods, principally due their N group [22]. Annually, the production of this type of compounds is about 700,000–10,000,000 t [23,24,25], and ≈280,000 t of dyes are released into water bodies and end in traditional wastewater treatment plants [23,24] where they are considered urban wastes and treated with primary and secondary treatments but only reaching 20% and 60% of removal, respectively [26]. These substances can cause negative and counterproductive effects in water bodies, contaminating the ecosystem and the species that inhabit it [27]. This not only aesthetically affects an ecosystem but also causes a serious disturbance in photosynthetic activity due to the low penetration of solar radiation, progressing directly to living organisms [28,29,30]. As a result of this, is necessary to find a suitable method for the removal or degradation of substances.

The most appropriate treatments against recalcitrant compounds are advanced oxidation processes (AOPs) mainly by their generation of a powerful chemical agent, the hydroxyl radical (•OH, 2.8 eV oxidative potential) [31,32,33]. Heterogeneous photocatalysis, one of the AOPs, normally uses TiO₂ as a catalyst and UV radiation; this generates gaps in electron pairs, which reach and break bonds of molecules [34].

Figure 2 shows the general mechanism of heterogeneous photocatalysis. The reaction in Equation (1) takes place when a beam of light containing photons with a wavelength less than or equal to 387 nm (UV-A region of the electromagnetic spectrum) falls on the surface of the photocatalyst, generating the electron–hole pair (${\mathrm{e}}_{\left(\mathrm{CB}\right)}^{-}, {\mathrm{h}}_{\left(\mathrm{VB}\right)}^{+}$) due to the migration of electrons from the valence band (VB) to the conduction band (CB).

$${\mathrm{TiO}}_2+\mathrm{hv}\to {\mathrm{TiO}}_2+{\mathrm{e}}_{\left(\mathrm{CB}\right)}^{-}+{\mathrm{h}}_{\left(\mathrm{VB}\right)}^{+}$$

(1)

The main equations of the mechanism for reactive radical species generation by photocatalysis are shown in the Section (S1) in Supplementary Materials.

The most active species responsible for carrying out the oxidation in the photocatalytic process are the •OH, superoxide radical (•O²⁻), and the hydroperoxide radical (•HO₂). Radical •OH has a redox potential of 2.8 eV, being greater than other reactive species such as O₃ (2.1 eV), Cl (2.03 eV), H₂O₂ (1.8 eV), and CO₃⁻ (1.59 eV), among many others [35]. The OH redox potential is greater even than that generated by the holes (h⁺) produced in the photocatalyst band jump (2.53 eV) [36,37]. These reactive species can degrade pollutants in a fast and non-selective manner until mineralization to produce non-toxic substances [38]. The great oxidizing power of •OH makes it capable of degrading most organic and organometallic pollutants until their mineralization by transforming them into CO₂, water, and inorganic ions [39]. Photocatalysis has been used to degrade this type of recalcitrant substances, obtaining positive results [40].

There are different types of reactors for wastewater treatment by heterogeneous photocatalysis, but they all have an exclusively designated place in the system for fixing the semiconductor, which will receive UV radiation artificially, with a UV lamp, or natural solar radiation [41]. Flat plate photocatalytic reactors using natural solar radiation have been recognized as a suitable alternative, which is largely due to their low manufacturing and maintenance cost. The countries in the planet’s Sunbelt are the most appropriate for using this type of technology with natural solar radiation, given that they receive an average annual solar irradiation of 5.5 kWh/m²/day, and on occasions, it can reach 6.5 kWh/m²/day [42].

Flat plate solar reactors used in heterogeneous photocatalysis have been receiving great interest as an alternative application due to their low cost of manufacture and maintenance [43]. However, the main challenge of these reactors lies in having enough solar radiation to penetrate and distribute itself over a large contact surface of the photocatalyst. Freudenhammer et al. [44] report results of a pilot study with fixed bed and thin film reactors (TFFBR) for the treatment of wastewater from the textile industry.

Non-concentrated flat plate reactors have significant advantages and simple structure, because they do not use mechanical parts or solar-tracking devices, they have a low production cost, and they are easier and cheaper to install compared to systems with concentration; however, their design must be carefully thought out with the requirements for chemical stability, weather resistance, and UV light in mind [45,46]. Since these reactors do not have a photon concentrator, diffuse light can be captured without the need for a reflective surface, which eliminates the loss of efficiency that other types of reactors present [47].

The aim of this work was to degrade the MTC compound by photolysis and heterogeneous photocatalysis in a flat plate reactor, with TiO₂ as a catalyst and solar radiation as the activation energy. In addition, the effects of different pH levels and doses of hydrogen peroxide (H₂O₂) on the degradation efficiency of MTC were evaluated by kinetic and statistical analysis, comparing the MTC degradation efficiency. Collector area per order (A_CO)(m²/m³–order) was also estimated for the evaluation of AOPs in aspects of energy requirements or area required.

2. Results and Discussion

According to Dean et al. [42], in low MTC concentrations, the dimer formation does not take place. The initial concentration of MTC to perform the experiments was 30 mg/L, which was selected in order to avoid dimer formation, as such a formation could intervene with absorbance reading. As the absorbance reading at 670 nm was used to calculate the MTC concentration on samples, the method could have been affected for the absorbance of 605 nm, which is the maximum absorbance for MTC dimers [48].

According to Houas et al. [49], one hour is required to reach MTC and TiO₂ adsorption equilibrium; as experiments lasted for only 60 min, it was important to reject the possibility that only the adsorption of MTC molecules on the TiO₂ surface (on photocatalysis experiments) was to be blamed for the degradation results. Such a possibility was rejected by the results of control experiments.

Control experiments lasted for 60 min, similarly to regular experiments, but in the absence of radiation. The results were always lower than 10%, so only the absorbance of MTC on the photocatalyst or support was rejected as the explanation for the degradation.

It is known that in basic solutions, MTC can transform to trimethylthionine [48]; such a transformation was discarded by the MTC concentration reading in control experiments, which were performed in the absence of radiation. Trimethylthionine is a phenothiazine derivative dye that presents redox activity [50].

When H₂O₂ was added to control experiments, MTC degradation was not remarkable; these results allowed authors to discard oxidant presence as the main factor acting in the processes. The highest degradation by H₂O₂ addition in control photolysis experiments was 6% in a neutral pH.

2.1. MTC Degradation by Photolysis

The photolytic degradation of MTC ranged from 3.27% to 68.68% after 60 min of experimentation. Of the three pH magnitudes tested, acidic pH yielded the best results, which are portrayed in Figure 3. Actually, at the higher pH, the degradation was lower (

Table 1. MTC degradation percentage and kinetic parameters calculated for photolytic and photocatalytic processes under three different pH (3.5, 6.5, and 9) and H₂O₂ doses (no dose, 0.5, and 1 mM/L).

		Photolysis				Photocatalysis
pH	H2O2 (mM/L)	Kph (min−1)	Error	τ1/2 (min)	Degradation (%)	KphC (min−1)	Error	τ1/2 (min)	Degradation (%)
3.5	0	0.0199	0.000538	34.90	68.68	0.0340	0.000704	20.37	86.68
	0.5	0.0151	0.001216	45.76	56.14	0.0255	0.002014	27.18	75.22
	1	0.0143	0.000867	48.42	56.26	0.0276	0.002333	25.15	76.26
6.5	0	0.0121	0.001125	57.36	45.56	0.0222	0.003326	31.26	63.73
	0.5	0.0113	0.001537	61.24	41.22	0.0225	0.001575	30.80	69.44
	1	0.0103	0.001493	67.55	39.43	0.0153	0.001542	45.35	53.77
9	0	0.0015	0.000379	470.20	6.28	0.0181	0.002466	38.30	57.17
	0.5	0.0093	0.001157	74.16	36.53	0.0093	0.001146	74.18	38.43
	1	0.0012	0.000359	599.80	3.27	0.0043	0.000504	160.71	18.74

).

Figure 3 portrays the variation of concentration (mg/L) at each time for the degradation of MTC in aqueous solution when treated by photolysis in a solar photocatalytic reactor (SPR), without photocatalyst, at different pH and without H₂O₂ addition. As shown in Figure 3, degradation of 68.68% was achieved under acidic pH, being the highest degradation reached in this experimental series.

When photodegradation experiments take part using a UV-C lamp as the source of radiation, H₂O₂ addition improves results, given the formation of •OH radicals after peroxide breakup [51]; in this case, as experiments were performed using solar radiation, H₂O₂ addition actually hinders MTC degradation.

Given that H₂O₂ addition in control experiments does not show degradation, but in experiments in the presence of radiation, it actually hinders the degradation, the authors believe this to be an indication of competence for radiation between MTC and peroxide molecules.

Zhou et al. [52] degraded only 13% under pH 7, when solar radiation was applied to a MTC solution with a 400 mg/L concentration. For near-neutral pH, degradation in this study reached 45%; nevertheless, comparison is difficult, as the initial concentration here considered is lower (30 mg/L).

Djellabi et al. [53] reported in their study a low MTC degradation, reaching 9% after 5 h of experiments, in acid pH condition (2.7) and 10⁻² M of H₂O₂, but with solar sunlight radiation intensity below that reported in this study.

In near-neutral pH (6.5), a maximum degradation of 45% of MTC was obtained without oxidant agent addition, 41.22% with 0.5 mM, and 39.43% when 1 mM was added. These results are proximate to those reported by Benhabiles et al. [54], where their photolysis experiments reached 35% contaminant degradation, in similar pH conditions, without H₂O₂ addition, and solar irradiation above 650 W/m².

Conversely, in basic pH condition, the best degradation percentage of MTC was obtained when 0.5 mM of H₂O₂ was added, reaching 36.53%, but without the addition and dose of 1 mM, presenting 6.28% and 3.27%, in that order.

2.2. Solar Heterogeneous Photocatalysis to Degrade MTC

Experiments performed in the presence of photocatalyst were always better than those of photolysis (Table 1). The highest degradation was reached in a photocatalysis experiment, without H₂O₂ addition and in acidic pH. In these experiments, as in photolysis (Section 2.1), H₂O₂ addition hinders degradation but only for acidic and basic pH. Under neutral pH, the addition of 0.5 mM/L of peroxide slightly improves degradation.

The results of degradation of MTC by solar heterogeneous photocatalysis without H₂O₂ addition on three different pH magnitudes are portrayed in Figure 4. As can be noted, the best degradation was achieved on acid pH (86.68%), which is followed by near neutral pH (63.7%) and basic pH (57.1%). Chaudhari et al. [55] reported a maximum of 65% degradation of MTC with TiO₂ as a photocatalyst in 2 h of reaction and similar pH and solar irradiation conditions.

In the other hand, in neutral pH conditions, the best degradation (69.44%) was obtained when 0.5 mM of H₂O₂ was added, but as can be seen, this result is lower than found in acid pH. Meanwhile, when MTC was treated in basic pH conditions, a degradation of 18.74% was obtained with 1 mM H₂O₂ addition, 38.43% when 0.5 mM was added, and 57.17% without dosage of oxidant agent (Table 1).

In 2015, Benhabiles et al. [54] reported a 98% degradation of MTC after 120 min of reaction in pH 11, but, as can be seen, our results yield a different conclusion: the lowest results for MTC degradation were obtained in basic pH. In agreement with what was reported by Zaruma et al. [51], it was possible to observe that in higher pH, there exists a saturation and coloration of the catalyst impregnated in the glass plate, which the authors believe is an explanation for low degradation results under basic pH.

2.3. Kinetic Analysis Results

The kinetic constants (K_ph and K_phC) and half lifetime (τ_1/2) were calculated for experiments performed with the addition of H₂O₂ and in the absence of this oxidant agent. Kinetic data are presented in Table 1, considering first-order reaction for photolysis and photocatalysis experiments after 60 min of experimentation, under the three pH values tested.

The hindering of degradation reactions could be caused by an H₂O₂ addition in a too high dose. According to Wang and Xu [56], H₂O₂ addition increases degradation up to a certain point, but when the peroxide presence is too high, it inhibits the photolytic degradation of organic compounds.

It is well known that MTC is photosensitized under visible light illumination, in a process where the molecule is excited instead of the photocatalyst, as shown in Equation (2) [56]. In photolysis experiments, the degradation depends upon photosensitization of the MTC, while in photocatalysis, a combination of both processes takes place: radiation excites both molecules, the UV portion of solar radiation excites TiO₂, while the visible portion excites the MTC directly, so the total spectrum of solar radiation is used, as shown in Equation (3).

$$\mathrm{MTC}+\mathrm{hv}\left(\mathrm{visible}\mathrm{light}\right)\to {\mathrm{MTC}}^{*}$$

(2)

$${\mathrm{TiO}}_2+\mathrm{hv}\left(\mathrm{UV}\mathrm{radiation}\right)\to {\mathrm{e}}_{\mathrm{cb}}^{-}+{\mathrm{h}}_{\mathrm{vb}}^{+}$$

(3)

Excited MTC can also react with TiO₂ in photocatalytic processes, as shown in Equation (4):

$${\mathrm{MTC}}^{*}+{\mathrm{TiO}}_2\to {\mathrm{MTC}}^{+}+{\mathrm{TiO}}_2\left({\mathrm{e}}^{-}\right).$$

(4)

Given this, better degradation was expected on photocatalysis experiments, as both pathways for MTC degradation are taking place, and more of the solar radiation spectrum is useful for degradation.

Zaruma et al. [51] reported in their study that using a UV-C lamp as a radiation source and under pH of 3.5, better results were reached in absence in the photocatalyst, and they explained their results as a product of the H₂O₂/UV-C interaction, given that H₂O₂, when irradiated with UV-C radiation, yields OH radicals [57]. As this study reports results from solar processes, no UV-C radiation reached H₂O₂ molecules, discarding this pathway for degradation and thus explaining that H₂O₂ addition did not improve results.

In the past, electrostatic interactions between photocatalyst and pollutant have been reported to play a role in photocatalysis experiments [58]; as MTC is a cationic molecule [49,59], better results from photocatalysis experiments were expected from basic pH when TiO₂ is negatively charged [60,61,62]. Instead, such electrostatic attraction appears to be detrimental for degradation, given a possible color saturation by the photocatalyst as, according to Houas et al. [49], the adsorption of MTC on TiO₂ increases as pH increases. However, again, in control photocatalysis experiments, MTC degradation was never higher than 10%, so the adsorption of MTC on the photocatalyst was discarded as the mechanism behind degradation shown in Table 1.

2.4. Statistical Analysis Results

Student’s t-test, which was used to compare the reaction rate constants from both photolysis and photocatalysis experiments, showed that in almost all the cases of pH conditions and H₂O₂ addition, rates are different from each other (p < 0.05), but not the ones obtained under pH 9 and with 0.5 mM/L of the oxidant agent addition. These results coincide with the percentages of degradation reported in Table 1; in these conditions, the degradation percentages reached for photolysis and photocatalysis are very close (36.53% and 38.43%, respectively). Student-t analysis allowed authors to assure that both processes, when compared under the same conditions, throw statistically different results.

ANCOVA analysis, where MTC degradation was the response variable, showed significant differences (p < 0.05) for all the samples regarding the process (photolysis and photocatalysis) and pH level (acidic, neutral, and basic) (Figure 5). For the H₂O₂ factor (0, 0.5, and 1 mM/L), significant differences were found in times of 10, 20, 30, and 60 min.

In the experiments, MTC degradation was found to be dependent of the applied process, H₂O₂ presence, and pH of the solution.

2.5. Collector Area per Order (A_CO) Estimation

A comparative estimation of the A_CO, (m²/m³–order) for MTC degradation by photolysis and heterogeneous photocatalysis processes is shown in

Table 2. Estimation of ${\mathrm{A}}_{\mathrm{CO}}$, (m²/m³–order) and comparative efficiency for both photolytic and heterogeneous photocatalytic processes, $\mathsf{\epsilon }=[({\mathrm{A}}_{{\mathrm{CO}}_{\mathrm{photol}}}-{\mathrm{A}}_{{\mathrm{CO}}_{\mathrm{photocat}}})/({\mathrm{A}}_{{\mathrm{CO}}_{\mathrm{photol}}})]\times 100$, for MTC degradation, based on a first-order reaction (n = 1), taking place in a batch reactor and a low concentration (30 mg/L), in acidic conditions (pH 3.5), in experiments at noon.

	Photolysis	Photocatalysis	Efficiency (ε)
H2O2 (mM/L)	${\mathrm{A}}_{\mathrm{C}\mathrm{O}}$(m2/m3–Order)	${\mathrm{A}}_{\mathrm{C}\mathrm{O}}$(m2/m3–Order)
0	97	56	42.26%
0.5	137	81	40.87%
1	136	78	42.64%

, utilizing the method for solar energy-driven systems based on a first-order reaction (n = 1), taking place in a batch reactor and a low concentration [63]. Table 2 shows the effect of the H₂O₂ dose or not on A_CO estimation for both photolysis and heterogeneous photocatalysis processes at pH 3.5, under the same experimental conditions and at noon, following Equation (9).

Bandala and Estrada [64] reported similar ${\mathrm{A}}_{\mathrm{CO}}$ magnitudes to the ones here presented for comparison of four different solar collection geometries. In all cases, for application to the photocatalytic degradation of oxalic acid, the collection area was 0.75 m², where TiO₂ was employed as the catalyst by homogeneous photocatalysis, and the solution was recirculated at 168 L/: parabolic trough concentrator (PTC), ${\mathrm{A}}_{\mathrm{CO}}=25$ m²/m³–order; V trough collector (VTC), ${\mathrm{A}}_{\mathrm{CO}}=38$ m²/m³–order; compound parabolic concentrator (CPC), ${\mathrm{A}}_{\mathrm{CO}}=34$ m²/m³–order, when TiO₂ concentration is 0.01 g/L, and flat tubular (FT), ${\mathrm{A}}_{\mathrm{CO}}=35$ m²/m³–order, when TiO₂ is 0.05 g/L.

The lower the collector area per order, the higher the efficiency of the process. In this regard, ${\mathrm{A}}_{\mathrm{CO}}$ estimation (m²/m³–order) shows better magnitude for MTC solar heterogeneous photocatalytic degradation than that from photolysis (Table 2), so the photocatalysis efficiency is always higher (ε > 40%) in all experiments performed in pH of 3.5, with and without H₂O₂ addition. The best ${\mathrm{A}}_{\mathrm{CO}}=56$ m²/m³–order was obtained for photocatalysis without H₂O₂ (Table 2). These results are in agreement with the MTC kinetic degradation reported in Table 1, confirming the better efficiency for the photocatalysis process to the one from photolysis.

3. Materials and Methods

3.1. Flat Plate Solar Reactor

A SPR (Figure 6) with a metal base and an acrylic plastic container was used, which supports a flat glass plate with a contact area of 0.1 m² (0.3 m × 0.33 m) and has a slope of 20° with respect to the horizontal. The SPR was adjusted to Durango City, México latitude (24°1′40″ N), so it can receive the greatest amount of sunlight [65]. The reactor maintains a constant flow (127 L/h) in the laminar regime (Re < 1000), recirculating the solution from a collection tank [66].

3.2. Process and Experimental Design for the Study

Experiments were carried out for the degradation of MTC (HYCEL, Zapopan, Jal., Mexico) in aqueous solution by photolysis and photocatalysis with TiO₂ (Fermont, Monterrey, N.L., Mexico) as the photocatalyst, which was impregnated by direct spraying on the glass plate, using the method proposed by Stintzing [67].

In relation to the initial concentration of MTC to be degraded, the work of Stintzing was taken into account [67], where a concentration of 25 g/m² de TiO₂ was optimal for the degradation of MTC at an initial concentration of 250 mg/L, which is a relationship similar to that used in this work where the concentration of the photocatalyst was 2 g/m² in order to degrade 30 mg/L of MTC. The previous concentration (2 g/m²) has shown to be the optimal photocatalyst mass-support surface ratio used by this research group, since it presented the best results for the degradation of organic pollutants such as 2,5-dichlorophenol [31,41,68], and cytostatic contaminants such as 6-mercaptopurine [37]. It has also proven its effectiveness for the inactivation of fecal coliforms [58].

Table 3. Degradation of MTC and other dyes by heterogeneous photocatalysis with different concentrations of TiO₂ on various supports.

Support	Catalyst	TiO2 (g/m2)	Dye Initial Concentration	Radiation	Degradation (%)	Ref.
Concrete	TiO2-P25	25	3MTC	Solar radiation	80	[67]
Glass slides	TiO2-P25	2.8	4BDR (50 mg/L)	UV lamp	94	[69]
			5YCDG (50 mg/L)		89
Glass plate	TiO2	2.8	6DY (25 mg/L)	UV lamp	~60	[70]
			DY (25 mg/L)	Solar radiation	~45
			7DB (25 mg/L)	UV lamp	~80
1PP	TiO2-P25	0.35	MTC (4 mg/L)	UV lamp	~90	[71]
Glass plate	Titanium (IV) oxide (99% anatase)	6.5	MTC (20 mg/L)	UV-vis lamp	96.1	[72]
		39			98.68
2ACF	TiO2/ACF	4	MTC (40 mg/L)	UV lamp	99	[73]
Glass plate	P25-TiO2/ENR/PVC composites	15	MTC (12 mg/L)	UV lamp	99.49	[74]

¹ polpropylene, ² actived carbon fiber, ³ methylene blue, ⁴ black DR, ⁵ yellow CDG, ⁶ drimarene yellow, ⁷ drimarene black.

shows the proportions of TiO₂ photocatalyst on different supports (g/m²) for the degradation of MTC and other dyes. Alinsafi et al. [69] and Zahraa et al. [70] used a concentration of photocatalyst (TiO₂) slightly higher than that used in this work, with 2.8 g/m², while Maleki and Bertola [71] used a considerably lower proportion with 0.35 g/m².

Two processes were considered for the experimental design: photolysis and photocatalysis; three pH levels: 3.5, 6.5, and 9; in the same way, three doses of H₂O₂ (0, 0.5, and 1 mM/L) were tested to verify the effect of the addition of oxidizing agent.

Each experiment lasted 60 min, with a volume of 2 L of sample and concentration of 30 mg/L of MTC, using distilled water for the preparation of the solutions. The experiments were performed under three pH magnitudes (3.5, 6.5, and 9) to check the pH effect on the contaminant degradation, and the initial pH was adjusted adding NaOH or HNO₃ solutions.

Similarly, the effect of an oxidizing agent was analyzed by adding different doses of hydrogen peroxide: no dose, 0.5, and 1 mM H₂O₂/L, which were added to the solution after pH adjustment. For the experiments without H₂O₂ dose, the experimental time began when the solution covered the whole glass plate contact area; for the experiments with H₂O₂ dose, the time began after the solution reached the whole contact area of the plate and the H₂O₂ was added.

Samples were taken at times of 5, 10, 15, 20, 30, 45, and 60 min. MTC degradation was followed by UV-Vis spectrophotometry at a wavelength of 670 nm for each experiment [75]. To ensure that H₂O₂ addition is not the only MTC degradation contributing factor, a series of control experiments were performed in the absence of radiation (under shadow) at different pH values.

Radiation, humidity, and temperature data were measured by the meteorological station of the Secretaría de Recursos Naturales y Medio Ambiente, which was measured by a WE300 Solar Radiation Sensor pyranometer (Global Water, Yellow Springs, OH, USA). Experiments were conducted on days with an absence of air currents, in an established time range 12:00–13:00 h, in cloudless days of April–May months, with an average solar radiation incidence 980 W/m², as shown in Figure 7.

A 2 × 3 × 3 factorial design with covariates and repeated measures was used to analyze the degradation results and is described through Equation (5):

$$\begin{gathered} \mathrm{Y}\mathrm{ijklm}=\mathsf{\mu }+{\mathrm{P}}_{\mathrm{i}}+{\mathrm{D}}_{\mathrm{j}}+{\mathrm{pH}}_{\mathrm{k}}+{\mathrm{t}}_{\mathrm{l}}+\left({\mathrm{a}*\mathrm{t}}_1\right)+\left({\mathrm{b}*\mathrm{t}}_2\right)+\left(\mathrm{c}*\mathrm{r}\right)+\left(\mathrm{d}*\mathrm{H}\right)+{\mathrm{C}}_0+{\left(\mathrm{P}*\mathrm{M}\right)}_{\mathrm{ij}} \\ +{\left(\mathrm{P}*\mathrm{pH}\right)}_{\mathrm{ik}}+{\left(\mathrm{P}*\mathrm{t}\right)}_{\mathrm{il}}+{(\mathrm{M}*\mathrm{pH})}_{\mathrm{jk}}+{(\mathrm{M}*\mathrm{t})}_{\mathrm{jl}}+{\left(\mathrm{pH}*\mathrm{t}\right)}_{\mathrm{kl}}+{\mathsf{\epsilon }}_{\mathrm{ijklm}}. \end{gathered}$$

(5)

In Equation (5), the response variable corresponds to Y_ijklm, the general average of the model is µ, P_i is the process (photolysis P_i = 1 and photocatalysis P_i = 2), D_j is the H₂O₂ dose (no dose D_j = 1, 0.5 mM D_j = 2, and 1 mM D_j = 3), pH_k is the pH level (3.5, 6.5, and 9), C₀ is the initial concentration, r is the intensity of solar radiation, t₁ and t₂ correspond to the initial and final temperature, H is for humidity, and ε_ijklm represents the error.

3.3. Kinetic Analysis

The experimental data kinetic analysis was performed considering a first-order reaction (n = 1) [46]. Equation (6) was utilized to calculate the degradation rate constant of the experiments [46,51]. More information about Equation (6) can be found in Supplementary Materials (Section S2).

$${\left(\mathrm{X}\right)}_{\mathrm{t}}={\left(\mathrm{X}\right)}_0{\mathrm{e}}^{-({\mathrm{K}}_{\mathrm{phC}})·\mathrm{t}}.$$

(6)

In Equation (6), X_t is the concentration of MTC over time, X₀ represents the MTC solution initial concentration, and K_phC corresponds to photocatalytic rate constant.

The photolytic constant (K_ph) was determined by formal kinetics [76], and the half-life time calculation (${\mathsf{\tau }}_{1/2}$) was obtained by Equation (7) [44]

$${\mathsf{\tau }}_{1/2}=\frac{\ln \left(2\right)}{\mathrm{k}}.$$

(7)

3.4. Statistical Analysis

Student’s t-test (Equation (8)) was used in order to compare the MTC degradation kinetic constants from photolysis and photocatalysis processes performed under the same initial pH and H₂O₂ addition. An analysis of covariance (ANCOVA) allowed authors to evaluate the effect of the factors considered by the experimental design, using an α = 0.05. ANCOVA was performed using the SAS Studio 9.0 (SAS Institute Inc., Cary, NC, USA) statistical package, and the Gauss–Markov assumptions were verified.

$$\mathrm{t}=\frac{{\mathrm{k}}_1-{\mathrm{k}}_2}{\sqrt{\frac{{\mathsf{\sigma }}_1^2}{{\mathrm{n}}_1}+\frac{{\mathsf{\sigma }}_2^2}{{\mathrm{n}}_2}}}$$

(8)

In Equation (8), t represents the Student’s t statistics, k₁ and k₂ represent the photocatalytic (K_phC) and photolytic (K_ph) reaction constants, respectively; variances are represented by ${\mathsf{\sigma }}_1^2$ and ${\mathsf{\sigma }}_2^2$, and the number of observations is represented by n₁ and n₂.

3.5. Solar Energy-Driven System

In this study, MTC pollutant degradation was carried out in a solar flat plate reactor; consequently, the standard figures of merit suggested by IUPAC [63] can be utilized for the evaluation of AOPs in the aspects of energy requirements or area required.

The collector area per order (${\mathrm{A}}_{\mathrm{CO}})$ (m²/m³–per order) is the collector area required to reduce the concentration in polluted water by one order of magnitude in a time t, in a unit volume (e.g., 1 m³), when the incident solar irradiance is 1000 W/m² [63]. ${\mathrm{A}}_{\mathrm{CO}}$ was calculated by Equation (9), using the method for solar energy-driven systems based on a first-order reaction (n = 1), taking place in a batch reactor and a low concentration [63]:

$${\mathrm{A}}_{\mathrm{CO}}=\frac{\mathrm{A}·{\overline{\mathrm{E}}}_{\mathrm{S}}·\mathrm{t}}{\mathrm{V}·\log \left(\frac{{\mathrm{c}}_{\mathrm{i}}}{{\mathrm{c}}_{\mathrm{f}}}\right)}.$$

(9)

In Equation (9), A_co is the collector area per order (m²/m³–order), A is the reaction area of the reactor, ${\overline{\mathrm{E}}}_{\mathrm{S}}$ is the average solar irradiance over the time of a given treatment, t is the treatment time, V is the unit volume treated of pollutant, and c_i and c_f are the influent and effluent pollutant concentrations.

4. Conclusions

In the absence of solar radiation, the experiments performed did not show remarkable MTC degradation, not even when H₂O₂ was added as an oxidant agent at the beginning of the experiment.

H₂O₂ addition in control experiments does not show degradation, but in experiments in the presence of radiation, it actually hinders the degradation; the authors believe that this is an indication of competence for radiation between MTC and peroxide molecules.

The experiments performed in the presence of a photocatalyst were always better than those of photolysis. The highest degradation (86.68%) was reached in a photocatalysis experiment without H₂O₂ addition and in acidic pH 3.5.

In photolysis experiments, the degradation depends upon photosensitization of the MTC, while in photocatalysis, a combination of both processes takes place: radiation excites both molecules, the UV portion of solar radiation excites TiO₂, while the visible portion excites the MTC directly. Better degradation was expected on photocatalysis experiments, as both pathways for MTC degradation are taking place, and more of the solar radiation spectrum is useful for degradation.

The ANCOVA results for MTC degradation fulfilled the Gauss–Markov assumptions and showed significant differences (p < 0.05) for all the times with respect to the process (photolysis and photocatalysis) and pH level (acidic, neutral, and basic).

${\mathrm{A}}_{\mathrm{CO}}$ estimation (m²/m³–order) shows better magnitude for MTC solar heterogeneous photocatalytic degradation than that from photolysis, so photocatalysis efficiency is always higher (ε > 40%) in all experiments performed at a pH of 3.5, with and without H₂O₂ addition.