Metronidazole Electro-Oxidation Degradation on a Pilot Scale

Abstract

In this investigation, metronidazole was degraded in an aqueous solution through electro-oxidation. A DiaClean^® cell was used to accommodate a stainless-steel electrode as a cathode and a boron-doped diamond (BDD) electrode as anode. This setup provides several electrochemical advantages, including low currents, a high operational potential, and, frequently, low adsorption compared to conventional carbon materials. The physicochemical parameters were estimated after 180 min of treatment, applying different current densities. The concentration of metronidazole was monitored by HPLC to assess degradation, resulting in 30.67% for 30 mA cm⁻², 79.4% for 50 mA cm⁻², and 100% for 100 mA cm⁻². The TOC mineralization percentages were 12.71% for 30 mA cm⁻², 14.8% for 50 mA cm⁻², and 29.9% for 100 mA cm⁻². Also, biodegradability indices of 0.70 for 30 mA cm⁻², 0.81 for 50 mA cm⁻², and 0.93 for 100 mA cm⁻² were obtained. The byproducts found were formic acid and acetic acid. A pseudo-first order kinetic model was thus obtained due to the quasi-stable concentration achieved through hydroxyl radicals, given that they do not accumulate in the medium, due to their high rate of destruction and short lifespan.

1. Introduction

Recently, the consumption of pharmaceutical products has increased considerably at a global level due to a significant population increase, which in turn has led to an overwhelming use of said products. As such, large concentrations of emerging contaminants have been detected in municipal water, surface water, groundwater reserves, drinking water, water treated in wastewater plants, and even in the ground [1,2]. Generally, only 30% of oral antibiotics are metabolized in the body, whilst the remnant 70% is directly excreted through urine and unabsorbed by the body [3]. Consequently, this has become a topic of vital importance, because the health of various living beings is affected by the introduction of these compounds into the environment [4,5]. This increases the resistance of microorganisms to antibiotics [6], making antibiotic removal or degradation a subject of great importance on a national level [7].

Pharmaceutical compounds from the nitroimidazole family like metronidazole (MTZ), because of their considerable water solubility [8] and their low biodegradability [9], are frequently used in the treatment of certain parasitic infections in human beings and animals. MTZ is highly active against a number of anerobic bacteria, bacteroides, and protozoa [10], and in some farms it is added to animal feed at subtherapeutic concentrations with the goal of increasing the animals’ weight [11,12]. In addition, MTZ is used as a feed additive for various poultry and fish to eliminate parasites. For this reason, farms and fish farms generate water with a high concentration of MTZ that can reach water bodies, causing imbalances in ecosystems. It has been reported that high concentrations of MTZ of 12.5 mg/L and 40.4 mg/L in microalgae, such as Chlorella sp. and Selenastrum capricornutum, present acute toxicity.

Although MTZ is an excellent broad-spectrum antibiotic, prolonged exposure to high concentrations has been reported to be carcinogenic due to its high toxicity. In addition, it may be mutagenic [12,13,14,15]. A genotoxic effect has also been found in human lymphocytes [16,17], as well as problems with the central nervous system [17].

MTZ contamination has been found at higher concentrations compared to other antibiotics, as is the case of a marine monitoring campaign at a fish farm in Spain, which showed that MTZ was found in concentrations of 13.4 ng L⁻¹, which was 30 times higher than that of other antibiotics [11]. In some hospital effluents a maximum concentration of MTZ of 9.4 μgL⁻¹ was found, while in treatments plants concentrations between 2–40 µg/L were found [12]. Also, some studies have reported that MTZ is recalcitrant due to the reported biochemical demand (BOD₅) of 4554 mg L⁻¹ and a concentration of 1000 mg L⁻¹ in real wastewater [14], which represents a high organic load and a risk for various organisms.

Unfortunately, large amounts of wastewater are discharged from domestic sources, hospitals, farms, and even illegal landfills by the pharmaceutical industry without prior treatment, which implies a risk of antibiotic residues and their metabolites [18,19]. Antibiotics such as MTZ are complex and recalcitrant molecules, so conventional methods with biological systems fail to degrade them and can even cause deactivation of the process.

For this reason, in recent years, advanced oxidation treatments like Fenton, photo-Fenton, ozonolysis, and electrochemical methods have become an excellent alternative for the degradation of these compounds [18,19,20,21,22]. It has been demonstrated that this is because, in comparison with other treatments, advanced oxidation processes are more efficient in the treatment of real contaminants at low concentrations from mg L⁻¹ to ng L⁻¹ [23], making them the most promising technology [18]. Hydroxyl radicals can degrade such organic and organo-metallic compounds through various mechanisms, the choice of which depends on the nature of the compounds [24].

Among the advanced oxidation procedures, the electrolytic processes have an advantage in being able to employ an ample range of electrodes, like those made of graphite, stainless steel, PbO₂, or SnO₂, and especially BDD (boron-doped diamond) electrodes [25], which are efficient in the degradation of various functional groups, including ammonia, cyanide, chlorophenols, and alcohols. It bears mentioning that boron-doped diamond electrodes currently represent a novel alternative due to their electrochemical behavior, because they can act as both anodes and cathodes, allowing a change in polarity, making them electrodes with potential capabilities for water disinfection treatments. Additionally, the use of electrodes in the DiaClean^® cell presents an advantage, because the cell setup (a reduced distance between the electrodes) accelerates the formation process of hydroxyl radicals, which allows the degradation of more complex molecules. In addition, the absence of cell partitions requires lower energy consumption compared to divided cells.

It is important to underline that, currently, the majority of results for the degradation of MTZ have been reported at the laboratory scale, employing small volumes from 200 mL [10] to 1 L [26]. However, it is important to consider implementation in a pilot plant, given that it is rarely possible to scale up degradation to a larger prototype [27], as scaling is a complicated aspect of the process due to not always obtaining the same efficiency and results as at the laboratory level. Thus, it is of the utmost importance to consider a favorable size to analyze events associated with increasing the volume, to allow a deeper understanding of the behavior of the degradation processes. Straightforwardly, the results presented here focus on the implementation of an advanced oxidation method to clarify the behavior of MTZ at the pilot plant scale. In the long term, this may lead to the development of the necessary technology to use these electrochemical methods as a pre-treatment in industries, hospitals, and farms. This would avoid the direct release of contaminants into treated effluents in conventional plants, thus reducing their environmental impact. As such, a pilot scale is a reasonably promising option, given that it could reduce costs in conventional treatment plants [28]. However, the most important methodological aspect is to confirm whether or not the degradation of MTZ is achieved, together with evaluating the reproducibility of this process; both of these are objectives that represent a challenge at higher scales [29,30].

The aim of this work was to improve MTZ removal from aqueous solutions at a pilot scale by adjusting the current density in a reaction system with recirculation through a DiaClean^® cell comprising a stainless-steel cathode and a boron-doped diamond anode (BDD).

2. Materials and Methods

Reagents. The deionized water that agrees with FEUM (Farmacopea de los Estados Unidos Mexicanos) specifications was bought from HYCEL (Schwechat, Austria). MTZ (

Table 1. Physical and chemical properties of MTZ [3,10,13].

Characteristic	Metronidazole (MTZ)	Units
Molecular formula	C6H9N3O3
Chemical structure		-
Molecular weight	171.2	g mol−1
Water solubility	9.5	g L−1, 25 °C
Melting point	159–163	°C
pKa	2.55	-
LogKow	−0.02	-
Koc	23	-
λ	320	nm
Vp	4.07 × 10−7	Pa

) BioXtra ≥ 98% and sodium sulphate ACS ≥ 99% anhydrous, granular were bought from Sigma Aldrich (St. Louis, MO, USA). Sodium sulphate ACS ≥ 99% anhydrous, granular, acetonitrile HPLC ≥ 99.9%, and sulfuric acid ≥ 96% were bought from Fermont (Monterrey, Mexico).

Experimental setup. The experiments were conducted at pilot-scale in a batch reaction system (Figure 1), which consists of a DiaClean^® cell model 101 equipped with a boron-doped diamond (BDD) anode and a stainless-steel cathode. It was manufactured by Adamant Technologies (La-Chaux-de-Fond, Switzerland) with an electrode surface area of 78.54 cm². The diamond coating had a boron content of 500 mg·dm⁻³, a thickness of about 2.83 μm, and a sp³/sp² ratio of 217. The cell was connected to a 20 L plastic tank working with 16 L of solution containing 30 mg L⁻¹ MTZ (considering the high concentrations found in wastewater), in which the solution was recirculated with a 286.92 L h⁻¹ flowrate and a retention time of 0.557 h, employing a peristaltic pump Masterflex BT Cole-Parmer Model 77111-6 (Vernon Hills, IL, USA) and a GW Instek Model GPR-1820HD (Taiwan, China) power source. The operation was performed under galvanostatic conditions, and the current densities were set within the 30–100 mA cm⁻² range, with an electrolyte support concentration of 0.05 M Na₂SO₄ at room temperature. The cell characteristics are presented in

Table 2. DiaClean^® cell.

Cell Characteristics
Electrode material	BDD/silicon anode Stainless steel cathode
Flow rate (L h−1)	286.92
Electrode geometry	Disc
Anode surface (cm2)	78.53
Feed tank volume (L)	16
Inner electrode gap (mm)	5

.

Characterization of pollutants. MTZ was quantified using two techniques. First, a spectrophotometer UV-Vis HACH model DR/4000UU (Loveland, CO, USA) was used to monitor the wavelength between 200 to 900 nm. Ultra-high-performance liquid chromatography (UHPLC) was the second technique. The UHPLC Thermo Scientific Vanquish chromatograph (Waltham, MA, USA) was equipped with a RefractoMax 521 Refractive Index Detector (Thermo Fisher Scientific, Waltham, MA, USA). Data analysis was performed employing a chromatography data system (CDS) using Thermo Scientific™ Chromeleon™ 7.0 software. An Eclipse XBD C-18 column (4.6 mm × 150 mm; 3.5 μm) was used. Each sample was filtered with Agilent (Santa Clara, CA, USA) brand 0.2 μm nylon acrodiscs before injection. For MTZ, 0.5 mL min⁻¹ of 40% acetonitrile and 60% water was used, the wavelength was 320 nm, and the injection volume was 20 µL. For carboxylic acid, a flow rate of 0.5 mL min⁻¹ of acidified water with 3 mM of sulfuric acid was used, with an injection volume of 20 µL at 60 °C, and an Aminex HPX-87H column (Hercules, CA, USA) (9 µm, 7.8 × 300 mm) [31].

The Total Organic Carbon (TOC) content was determined with TOC-L series of TOC analyzers (Shimadzu, Kyoto, Japan), with an ultra-wide range of 4 μg L⁻¹ to 30,000 mg L⁻¹, using a TOC-LCSH/CPH PC-Controlled model. This was done through the combustion catalytic oxidation method and infrared identification; each sample of 10 mL was filtered with Agilent brand 0.2 μm nylon acrodiscs before injection. The TOC concentration was determined according to a calibration curve [32].

Water quality determination of biochemical oxygen demand after five days (BOD₅) was determined in accordance with the parameters and method described in the International Standard ISO 5815 [33].

3. Results and Discussion

3.1. Pilot Scale of Degradation Kinetics

Figure 2 shows the behavior of MTZ degradation throughout the electrolysis process, employing a BDD anode and a stainless-steel cathode and modifying the current densities. For the experiments that were carried out, the hydroxyl radicals produced provoked degradation of MTZ, owing to their continuous formation during the entire process.

The electrolysis processes were carried out at current densities of 30 mA cm⁻², 50 mA cm⁻², and 100 mA cm⁻². Each of the processes involved a solution of 16 L with an initial concentration of 30 mg L⁻¹ of MTZ and an electrolyte support concentration of 0.05 M Na₂SO₄, at a pH of 7 ± 0.5 and at room temperature.

Figure 2 shows that a low current density (30 mA cm⁻²) induces slow degradation. Figure 2 also shows that, after 120 min treatment, a removal percentage of 17.9% at a current density of 30 mA cm⁻² was achieved. In contrast, the process carried out at a current density of 50 mA cm⁻² over the same amount of time achieved a removal percentage of 57.3%, representing an improvement of more than 50%. Finally, the process carried out at 100 mA cm⁻² over an identical time achieved 100% degradation of MTZ. Likewise, for 180 min, the process carried out at 30 mA cm⁻² achieved a removal percentage of 30.6% of MTZ, whilst over the same time period, a current density of 50 mA cm⁻² achieved a maximum elimination of 79.4%. Thus, the lowest applied current density of 30 mA cm⁻² causes the degradation process to be slower; meanwhile, at 50 mA cm⁻² and 100 mA cm⁻², complete degradation is possible in a shorter period.

A degradation percentage of 100% was achieved after treatment with 100 mA cm⁻², which is superior in comparison to other processes, such as UV/H₂O₂, which achieves elimination of only 41% [10]. This is also true when compared to coagulation, which has a degradation percentage of 89.3% for concentrations of 10 mg L⁻¹ [13] carried out at the laboratory scale. Electrolysis with a BDD anode and a stainless-steel cathode, when used for high volumes, can be efficiently employed in a pre-treatment process in industries or farms.

This degradation can be explained by the formation of hydroxyl radicals (Equation (1)) [3] on the surface of the electrode, subsequently reacting with the MTZ, forming degradation and mineralization products (Equations (2) and (3)) [22].

$$\mathrm{B}\mathrm{D}\mathrm{D}+{\mathrm{H}}_2\mathrm{O}\mathrm{B}\mathrm{D}\mathrm{D}{(}^{.}\mathrm{O}\mathrm{H})+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$

(1)

$$\mathrm{B}\mathrm{D}\mathrm{D}{(}^{•}\mathrm{O}\mathrm{H})+\mathrm{M}\mathrm{T}\mathrm{Z}\mathrm{B}\mathrm{D}\mathrm{D}+\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{b}\mathrm{y}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s}$$

(2)

$$\mathrm{B}\mathrm{D}\mathrm{D}{(}^{•}\mathrm{O}\mathrm{H})+\mathrm{M}\mathrm{T}\mathrm{Z}\mathrm{B}\mathrm{D}\mathrm{D}+\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}+\mathrm{i}\mathrm{n}\mathrm{o}\mathrm{r}\mathrm{g}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{c}\mathrm{s}$$

(3)

A degradation percentage of 100% was achieved after treatment with 100 mA cm⁻², which is superior in comparison to other processes, such as UV/H₂O₂, which achieves a degradation of only 41% [10]. Electrolysis with a BDD anode and a stainless-steel cathode, when used for high volumes, can be efficiently employed in a pre-treatment process in industries or farms [34,35].

It has been reported that more than 89.3% of MTZ was removed from 10 mgL⁻¹ samples by electrocoagulation on a laboratory scale [13]; however, it is well known that the contaminants remain in the generated sludge without any change in its chemical structure. Therefore, advanced oxidation processes have the advantage that they do not generate additional waste.

The increase in reaction rate (Figure 3) implies faster degradation of MTZ; upon applying a current density of 30 mA cm⁻², the specific reaction constant is 0.0019 min⁻¹. Thus, the process will evidently take longer to achieve 100% degradation; increasing the reaction speed means an increase in the production of subproducts.

Figure 4 shows the impact of the specific electric charged passed in the three processes. Despite the fact that the time of residence within the reactor is the same, it can be observed that, at a current density of 30 mAcm⁻², the degradation of MTZ decreases slowly, given that, during the 180 min in which the process is carried out, the MTZ is not completely degraded. This is in contrast to the processes carried out at current densities of 50 mAcm⁻² and 100 mAcm⁻², which contributes to the removal of MTZ.

Table 3. Pseudo-first order kinetic constants obtained at 100 mA cm⁻², 50 mA cm⁻², and 30 mA cm⁻².

Current Density (mA cm−2)	k1 (min−1)	r2	t1/2 (min)
100	0.0258	0.9641	26.8661
50	0.0083	0.9802	83.5117
30	0.0019	0.9721	364.8143

shows the constants obtained for the pseudo-first order kinetics. Figure 4 displays the tendency of the reaction rate for the processes carried out at 30 mA cm⁻², 50 mA cm⁻², and 100 mA cm⁻². Equation (4) shows the kinetics of the reaction, whilst the mass balance of the system is found in Equation (5). Thus, the influence of increasing the current density can be observed; the reaction speed at a current density of 100 mA cm⁻² is 0.0258 min⁻¹, achieving the greatest reaction rate of the three processes.

$$\mathrm{r}={\mathrm{k}}_1\left[\mathrm{p}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{u}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{t}\right]$$

(4)

$${\displaystyle \frac{\mathrm{d}\left[\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{i}\mathrm{d}\mathrm{a}\mathrm{z}\mathrm{o}\mathrm{l}\mathrm{e}\right]}{\mathrm{d}\mathrm{t}}}={-\mathrm{k}}_1\ast [\mathrm{M}\mathrm{T}\mathrm{Z}]$$

(5)

3.2. Treatment Quality Analysis

Figure 5 shows the Total Organic Carbon (TOC) at the beginning and the end of each process. In the graph it is observed that the reduction in TOC was relatively low in all cases. For the treatment of 30 mA cm⁻², a decrease of 12.71% was obtained; for the process conducted at 50 mA cm⁻², the decrease was 14.8%; and for a current density of 100 mA cm⁻², 29.9% was reached, all after 180 min of treatment. It is evident that low mineralization can be associated with the fact that the concentration of MTZ is high and confirms that the molecule is recalcitrant [15].

On the other hand, it can also be associated with the fact that, during generation of ^•OH, these radicals can react with the ${\mathrm{S}\mathrm{O}}_4^{-2}$ present in the electrolyte support, forming oxidizing species (Equations (6)–(9)) that help degradation; however, its oxidizing power is not as potent as that of ^•OH [32].

$${\mathrm{S}\mathrm{O}}_4^{-2}\to {\mathrm{S}\mathrm{O}}_4^{•-}+{\mathrm{e}}^{-}$$

(6)

$${\mathrm{H}\mathrm{S}\mathrm{O}}_4^{-}+\mathrm{O}{\mathrm{H}}^{•}\to {\mathrm{S}\mathrm{O}}_4^{•-}+{\mathrm{H}}_2\mathrm{O}$$

(7)

$${\mathrm{S}\mathrm{O}}_4^{-2}+{\mathrm{S}\mathrm{O}}_4^{-2}{\to {\mathrm{S}}_2\mathrm{O}}_8^{-2}$$

(8)

$${\mathrm{S}\mathrm{O}}_4^{•-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{S}\mathrm{O}}_4^{-2}+\mathrm{O}{\mathrm{H}}^{•}+{\mathrm{p}}^{+}$$

(9)

For the above reasons, analysis of BOD and COD was carried out, and the results are presented below.

The Biochemical Oxygen Demand (BOD₅) was determined because it is a good indicator of water quality (Figure 6) at the beginning and end of the process. This is because, by measuring the quantity of dissolved oxygen that is consumed by the microorganisms in order to break down the organic material dissolved in the samples, the efficiency of the metronidazole degradation process is indicated. In Figure 6, it can be observed that, before applying a current density of 30 mA cm⁻² (●) to the solution with an initial BOD₅ of 36.03 mg O₂ L⁻¹, in accordance with International Standard ISO 5815, it is classified as contaminated water; after this process, the concentration of BOD₅ decreases by 30.67%, makes it acceptable water with an indication of contamination. Meanwhile, 50 mA cm⁻² (△) had an initial BOD₅ indicator of 36.833 mgO₂ L⁻¹; a reduction of 74.01% in BOD₅ was achieved, after which the water would be considered by International Standard ISO 5815 to be non-contaminated water. Meantime, a current density of 100 mA cm⁻² (■) delivered a reduction of 95.77%, which according to International Standard ISO 5815 achieves a classification of ‘not contaminated water’.

In order to calculate the biodegradability index, the BOD₅/COD relationship was employed for wastewater [36], in which >0.8 indicates that it is highly biodegradable, a range of 0.7–0.8 indicates that it is biodegradable, 0.3–0.7 indicates low biodegradability, and <0.3 indicates that it is not biodegradable.

For this reason, Figure 7 shows that synthetic water contaminated with MTZ demonstrates low biodegradability, which confirms that the molecule is recalcitrant.

In Figure 7, it can be observed that the biodegradability index varies during the electrolysis process employing a boron-doped diamond anode and a stainless-steel cathode. For a current density of 30 mA cm⁻², the biodegradability index at the beginning of the process is 0.67, indicating low biodegradability, whilst at the end of the process, it increased to 0.70, becoming biodegradable. For the current density of 50 mA cm⁻², the initial index was 0.67, and the final index was 0.81; for 100 mA cm⁻², the initial index was also 0.67, but the final index was 0.93. In each process, it can be observed that the final biodegradability index after the electrolysis process shows the need for electrochemical pretreatment in order to facilitate processing in conventional treatment plants.

Figure 8 shows the pH increase occurring during the process, although in most electrochemical oxidation processes the concentration of protons tends to increase and thus decrease the pH due to the formation of various low molecular weight carboxylic acids such as acetic acid, formic acid, and oxalic acid [37]. When starting the degradation process of MTZ, the nitrogen molecules in its structure cause denitrification, which leads to the production of nitrates and nitrites (Equations (10)–(15)) [38,39], increasing the pH. In addition, persulphates can be produced at the cathode during the process, due to the presence of sodium sulphate as a supporting electrolyte, considering that sodium sulphate offers several advantages such as low cost, stability, safety, and compatibility with electrode materials, among others (Equations (6)–(9)) (Amado-Piña et al., 2022 [32]).

$${\mathrm{N}\mathrm{O}}_3^{-}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {\mathrm{N}\mathrm{O}}_2^{-}+{2\mathrm{O}\mathrm{H}}^{-}$$

(10)

$${\mathrm{N}\mathrm{O}}_3^{-}+{6\mathrm{H}}_2\mathrm{O}+8{\mathrm{e}}^{-}\to {\mathrm{N}\mathrm{H}}_3+{9\mathrm{O}\mathrm{H}}^{-}$$

(11)

$${\mathrm{N}\mathrm{O}}_3^{-}+{3\mathrm{H}}_2\mathrm{O}+5{\mathrm{e}}^{-}\to {{\displaystyle \frac{1}{2}}\mathrm{N}}_2+{6\mathrm{O}\mathrm{H}}^{-}$$

(12)

$${\mathrm{N}\mathrm{O}}_2^{-}+{5\mathrm{H}}_2\mathrm{O}+6{\mathrm{e}}^{-}\to {\mathrm{N}\mathrm{H}}_3+{7\mathrm{O}\mathrm{H}}^{-}$$

(13)

$${2\mathrm{N}\mathrm{O}}_2^{-}+{4\mathrm{H}}_2\mathrm{O}+6{\mathrm{e}}^{-}\to {\mathrm{N}}_2+{8\mathrm{O}\mathrm{H}}^{-}$$

(14)

$${\mathrm{N}\mathrm{O}}_2^{-}+{2\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{N}\mathrm{O}}_3^{-}+{\mathrm{H}}_2\mathrm{O}+{2\mathrm{e}}^{-}$$

(15)

3.3. Metronidazole Degradation Byproducts

Figure 9 shows the products of MTZ degradation at different current densities, identifying formic acid and acetic acid through HPLC. Figure 9a depicts the identification of formic acid at a density of 30 mA cm⁻² for 120 min, at 50 mA cm⁻² for 30 min, and at 100 mA cm⁻² for 5 min. This matches previous reports in the literature regarding electrolytic processing [40]. It can be observed that, at a current density of 30 mA cm⁻², a small quantity of formic acid is generated, which is lower in comparison with the processes carried out at current densities of 50 mA cm⁻² and 100 mA cm⁻². For the process carried out with a current density of 50 mA cm⁻², it is observed that the generation of formic acid is directly proportional to the treatment time, which is associated with the formation of ^•OH on the electrode surface; a certain selectivity can be associated with this compound, since acidic is not detected at this current density.

On the other hand, it is evident that, when applying a current density of 100 mA cm⁻², the rate of formic acid production is twice as fast as at 50 mA cm⁻², increasing proportionally to 60 min, and then the concentration is kept constant at 0.05 mg L⁻¹. Figure 9b shows that acetic acid formation, detected after 100 min, increased proportionally until the end of treatment. This behavior is attributed to the high production of radicals generated, where molecules with two carbons, a result of degradation, can be oxidized and generate acetic acid (Figure 9b).

This indicates that, at lower current densities, there is selectivity in the process, in which ^•OH radicals are first generated in the active centers of the electrode, beginning to break bonds in the MTZ molecule, generating a lower quantity of subproducts. In accordance with various other studies, the presence of oxalic acid and oxamic acid is also expected [41].

Figure 10 shows a possible degradation route by means of electrolysis using a BDD anode and a stainless-steel cathode. As is observed in Figure 10a, the most complex intermediary products could be generated at 30 mA cm⁻², considering that low current density is not enough to break the bonds in the molecule; as is observed in Figure 10b, more intermediary products with fewer functional groups can be produced employing a current density of 50 mA cm⁻². Finally, in Figure 10c there are several byproducts generated, such as low weight carboxylic acids, that are easily obtained by increasing the current density to 100 mA cm⁻².

n the presence of hydroxyl radicals through denitrification of the molecule, we observe elimination of the nitro group at different stages of the process, in addition to the fact that the transition from alcohols to aldehydes, the transition from aldehydes to carboxylates, and decarboxylation can occur during the process. Thus, we observed the formation of various organic products, such as acetic acid and formic acid, because of degradation of the MTZ molecule. In addition, nitrites and nitrates were observed via ion chromatography, although they could not be quantified [26,41].

3.4. Cost Analysis

For this treatment, a cost analysis was conducted with the experimental data obtained. For Equation (16), P is power (W), I is current (mA cm⁻²), and V is voltage (volts); for Equation (17), E is electric consumption (kWh) and t is the time over which power is consumed (h); and for Equation (18), E_C is specific energy consumption (kWh m⁻³),$\overline{\mathrm{U}}$ is the average voltage (V), and V is the volume of the treated solution (L) [21].

$$\mathrm{P}=\mathrm{I}\mathrm{V}$$

(16)

$$\mathrm{E}={\displaystyle \frac{\mathrm{P}\mathrm{t}}{1000}}$$

(17)

$${\mathrm{E}}_{\mathrm{c}}={\displaystyle \frac{\overline{\mathrm{U}}\mathrm{I}\mathrm{t}}{3600\mathrm{V}}}$$

(18)

In addition to this, the power employed in each system (Equation (16)) was calculated, considering that each electrical current involves different electricity consumption (Equation (17)); for the system at 30 mA cm⁻², the power calculated was 282 W with an electric consumption of 0.846 kW in a period of three hours and a specific energy consumption (Equation (18)) [21] of 0.01468 kWh m⁻³; with 50 mA cm⁻² current, the power was 470.4 W, with 1.4112 kW h of electric consumption and a specific energy consumption of 0.024 kWh m³; while for the last treatment at 100 mA cm⁻², the power calculated was 942 W, with an electric consumption of 2.826 kW h and a specific energy consumption of 0.049063 kWh m⁻³.

Therefore, the cost for the system at 30 mA cm⁻² for 3 h is approximately $4.9108 MXN = $0.2739 USD, for the system at 50 mA cm⁻² for 3 h is approximately $8.178 MXN = $0.4543 USD, and for the system at 100 mA cm⁻² for 3 h is approximately $16.4028 MXN = $0.9113 USD; estimating that 1 USD = 18 MXN. Thus, even using a current density of 100 mA cm⁻² over a period of 3 h, the process cost will be less than approximately $1 USD.

These results show that electric consumption for treatment for 3 h is energetically viable, to implement DiaClean^® cells with a boron-doped diamond anode and a stainless-steel cathode as a pre-treatment of MTZ, before being discharged in water effluents, reducing the environmental impact.

Table 4 shows the comparison of lab-scale and pilot plant treatments; it is easy to observe considerable differences in carrying out the scaling up of the process, considering that the initial technology investment is often costly and complex, while in the long term, the benefits can be observed.

Pilot-scale plants consume more energy; however, they are able to treat high volumes of wastewater; therefore, in the future, it is important to optimize the processes, modifying the configuration of electrodes and reactor end cells to achieve commercial viability [42].

In addition to this, several studies have shown the advantages of employing continuous flow reactors in several industries to degrade not only MTZ, but also other pollutants present in wastewater, conducting some economic studies to implement these new technologies as pre-treatments in wastewater from different industrial sectors [43,44].

Table 4. Scaling comparison table [45,46].

Laboratory Scale	Pilot Scale
Low energetic consumption, considering volumes less than 1 L	High energetic consumption, because of the large volumes treated
Ideal operating conditions are considered for optimization	Optimization depends on the performance at scale and the cost of operation
Operating costs are low because of the scale	High operating costs for commercial viability

Table 4. Scaling comparison table [45,46].

Laboratory Scale	Pilot Scale
Low energetic consumption, considering volumes less than 1 L	High energetic consumption, because of the large volumes treated
Ideal operating conditions are considered for optimization	Optimization depends on the performance at scale and the cost of operation
Operating costs are low because of the scale	High operating costs for commercial viability

4. Conclusions

In this study, a solution volume of 16 L was treated, achieving degradation of the MTZ molecule when the current density was raised to 100 mA cm⁻². Despite achieving 100% degradation of the molecule, the same could not be said for mineralization within a time of 180 min, but upon achieving degradation, various byproducts such as formic acid, acetic acid, nitrites, and nitrates are produced, owing to the presence of nitrogen atoms in the molecular structure.

These compounds have an added value and can be recovered through other types of electrochemical processes such as electrodialysis. Achieving purification and separation of these compounds, making the process sustainable, as well as reducing operating costs, could help achieve one of the most important points of the circular economy, whereby 100% mineralization will no longer be an indispensable requirement of electrolysis processes. Additionally, this process is being optimized to obtain a higher concentration of formic acid as a purchased value product to be used in fuel cells.

On the other hand, it was possible to increase the biodegradability of MTZ at all applied current densities, with a result of high biodegradability at 50 and 100 mA cm⁻², according to the BOD5/COD ratio. This result suggests that water can be treated in plants that use conventional methods, such as biological systems.

Thus, this pilot prototype can be scaled up to a greater size with the intention of implementation as a pre-treatment for various effluents. Although the scaling up of the processes currently represents a challenge, due to the increase in operating costs due to the energy requirement of the process, the use of pilot plants can provide a guideline for scaling up to industrial levels, as an alternative to current environmental problems, so it is important to optimize the operating conditions of this type of electrochemical processes to achieve the viability of the process.