Sustainable Degradation of Acetaminophen by a Solar-Powered Electro-Fenton Process: A Green and Energy-Efficient Approach

Abstract

The presence of acetaminophen (ACTP) in aquatic environments has become a significant concern due to its environmental persistence and the potential formation of toxic transformation products. This study systematically compares the performance of three electrochemical advanced oxidation processes (EAOPs), electro-oxidation (EO), electro-Fenton (EF), and solar photo-electro-Fenton (SPEF), for the degradation and mineralization of ACTP in aqueous media using boron-doped diamond (BDD) electrodes. Reactions were conducted under varying operational parameters, including current densities (15–60 mA cm⁻²), initial ACTP concentrations (10–30 mg L⁻¹), and Fe²⁺ dosages. In the SPEF system, natural sunlight was utilized as the source of UV-A irradiation (30–35 W m⁻²). Among the evaluated processes, SPEF exhibited the highest degradation efficiency, achieving up to 97% ACTP removal and 78% chemical oxygen demand (COD) reduction within 90 min. High-performance liquid chromatography (HPLC) analysis identified phenol and catechol as major intermediates, suggesting a degradation pathway involving hydroxylation, aromatic ring cleavage, and subsequent oxidation into low-molecular-weight carboxylic acids. Kinetic modeling revealed pseudo-first-order behavior, with a maximum rate constant of 0.0865 min⁻¹ under optimized conditions determined via Box–Behnken experimental design. Additionally, SPEF demonstrated enhanced energy efficiency (~0.052 kWh gCOD⁻¹) and improved oxidant regeneration under solar radiation, highlighting its potential as an environmentally friendly and cost-effective alternative for pharmaceutical wastewater treatment. These results support the implementation of SPEF as a sustainable strategy for mitigating the environmental impact of emerging contaminants, especially in regions with high solar availability and limited technological resources.

1. Introduction

Currently, the increasing presence of persistent organic pollutants (POPs) in water bodies is causing significant concern in the scientific community due to their harmful effects on health and their poor treatability with conventional treatments [1,2,3,4]. Among these POPs, one of the most prominent is ACTP, due to its high global consumption, which is frequently found in considerable concentrations in urban and hospital wastewater [5,6].

ACTP is classified as an analgesic medication. When ingested, it is not completely metabolized. It is wholly or partially excreted without modifying its chemical structure. It then reaches conventional treatment plants, which are not designed to eliminate it [7].

This compound has been detected in concentrations ranging from ng/L to μg/L, and one of the primary concerns is that its partial degradation can produce more toxic or persistent compounds [6,8].

Advanced oxidation processes (AOPs) emerge as a promising solution to this type of problem, offering a viable alternative for the degradation of drugs [9,10]. These processes focus on the generation of highly oxidizing species, such as the hydroxyl radical (^•OH), with an excellent capacity for the mineralization of persistent organic pollutants (POPs) [11,12,13,14,15,16,17]. Among the AOPs, electrochemical processes such as electro-oxidation (EO), electro-Fenton (EF), and solar photo-electro-Fenton (SPEF) have been demonstrated to be efficient in removing ACTP, as shown in Equations (1)–(5):

$$\mathrm{M}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{M}({}^{•}\mathrm{O}\mathrm{H})+\mathrm{O}{\mathrm{H}}^{-}$$

(1)

$${\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to \mathrm{F}{\mathrm{e}}^{3+}+{}^{•}\mathrm{O}\mathrm{H}+\mathrm{O}{\mathrm{H}}^{-}$$

(2)

$$\mathrm{F}{\mathrm{e}}^{3+}+{\mathrm{e}}^{-}\to \mathrm{F}{\mathrm{e}}^{2+}$$

(3)

$$\mathrm{F}{\mathrm{e}}^{3+}+{\mathrm{H}}_2\mathrm{O}+\mathrm{h}\mathrm{v}\to {\mathrm{F}\mathrm{e}}^{2+}+{}^{•}\mathrm{O}\mathrm{H}+\mathrm{O}{\mathrm{H}}^{+}$$

(4)

$$\mathrm{F}\mathrm{e}(\mathrm{O}\mathrm{H}{)}^{2+}+\mathrm{h}\mathrm{v}\to {\mathrm{F}\mathrm{e}}^{2+}+{}^{•}\mathrm{O}\mathrm{H}$$

(5)

Recent studies have achieved good results using different anode materials, such as boron-doped diamond (BDD), Ti/Pt, Ti/PbO₂, and Ti/IrO₂-SnO₂-Sb₂O₅ electrodes, as well as various radiation sources, such as UV and solar radiation, to enhance ^•OH generation and improve process efficiency [18,19,20,21,22,23]. Other research has employed experimental designs, such as Box–Behnken and multivariate methodologies, to optimize operating conditions [18,24,25,26,27,28,29]. In our previous research, we applied EO, EF, and SPEF processes for the removal of various contaminants, including dyes used in the textile and tanning industries [26,30], pesticides [31], and treatment of real effluents [30]. In all cases, we have achieved good results.

Here, we present a proposal for an integrated approach to the degradation of ACTP using EO, EF, and SPEF in a BDD/graphite system for EO and BDD/BDD for EF and SPEF, evaluating different pH values, current densities, and Fe(II) concentrations. Additionally, the SPEF utilizes natural solar radiation as a complementary energy source to evaluate its impact on process efficiency.

Unlike other studies, this work emphasizes the efficient removal of the contaminant at low operating costs, utilizing solar radiation without the need for concentrating devices, while considering feasible scenarios for environments with technological and energy resource limitations. Analytical techniques, including UV-Vis spectrophotometry, high-performance liquid chromatography (HPLC), and chemical oxygen demand (COD), were employed to monitor ACTP and total organic matter. This approach enables a detailed assessment of degradation, mineralization, and byproduct formation, thereby contributing to the design of more sustainable and efficient treatments for pharmaceutical contaminants in water [32,33].

2. Materials and Methods

2.1. Materials

ACTP (acetaminophen, C₈H₉NO₂) degradations were performed using synthetic solutions prepared with ultrapure water (resistivity of 18.2 MΩ cm, Milli-Q system). The contaminant was purchased from Sigma-Aldrich (Darmstadt, Germany) with a 98% or higher purity. Anhydrous sodium sulfate (Na₂SO₄, Merck (Darmstadt, Germany), ≥99.5%) was used as the supporting electrolyte at two different concentrations: 0.05 and 0.1 mol L⁻¹. All materials were purchased from local stores in León, Guanajuato, Mexico.

In the EF and SPEF processes, ferrous sulfate heptahydrate (FeSO₄ 7H₂O, Sigma-Aldrich) was used as the Fe(II) source, with concentrations ranging from 0.1 to 0.5 mmol L⁻¹. The pH of the solutions was adjusted to values between 3.0 and 7.0 using 0.1 mol L⁻¹ H₂SO₄ or NaOH.

2.2. Experimental Setup

The tests were conducted in a 250 mL stirred-tank reactor, Figure 1, continuously stirring at 60 rpm and with a constant oxygen supply to the system using an AQUAKRILL model 1588 air pump. The current applied to the electrodes was supplied by a BK PRECISION MODEL 1665 (Yorba Linda, CA, USA) power source. The working solutions were maintained at room temperature (25 °C) using 0.05 M Na₂SO₄ as the supporting electrolyte, at a pH of 3.0, adjusted with a concentrated H₂SO₄ solution. The interelectrode distance was approximately 2 cm to reduce system resistance. Electro-oxidation and Fenton reaction tests were run to evaluate the system’s behavior. For EO, BDD, and graphite, electrodes were used as anode and cathode, respectively (BDD/Graphite), while for the EF and SPEF tests, the solutions were treated with boron-doped diamond (BDD) electrodes as anode and cathode (BDD/BDD). The SPEF processes were conducted on sunny days during the summer of 2023, with natural UV radiation (300–400 nm) measured at 30–35 W/m² using a Kipp & Zonen CUV 5 radiometer (Sterling, VA, USA). The assays were performed in triplicate, and the average data are reported with a 95% confidence interval.

2.3. Analytical Methods

A Cintra 1010 UV-Vis spectrophotometer (GBC, Melbourne, Australia) was used to monitor the residual concentration of ACTP by measuring the absorbance at a wavelength of 243 nm, corresponding to the drug’s maximum absorbance peak.

Ion-exclusion high-performance liquid chromatography (HPLC) using an Agilent 1260 Infinity (Santa Clara, CA, USA) series was employed to detect and quantify ACTP concentration, but with a Zorbax SB-C18 column (Santa Clara, CA, USA) (25 mm × 4.6 mm, five μm particle size). The mobile phase comprised acetonitrile and water (50:50 v/v, pH 3.0 with H₃PO₄) and detection at 243 nm. Elution was carried out using an isocratic method, with a flow rate of 1.0 mL min⁻¹ at a temperature of 30 °C. ACTP exhibited a retention time of approximately 4.35 min, while two additional peaks were identified, corresponding to its central intermediates: catechol (t_r ≈ 6.26 min) and phenol (t_r ≈ 3.29 min).

The quantity of oxidizable material was estimated through chemical oxygen demand (COD) assays, following the 5220D method outlined in Standard Methods [34]. Hach DRB200 digestion tubes (Naucalpan de Juárez, Estado de México, México) and low and mid-range COD premeasured tubes (0–150 and 0–1500 mgO₂ L⁻¹) were used.

2.4. Experimental Design

The investigation into the efficacy of SPEF involved the exploration of operational parameters, specifically the time of electrolysis (E_t), initial ACTP concentration (Co ACTP), and applied current density (j). A three-level Box–Behnken design was implemented, including 15 runs, each with 3 replicates, and randomized using a free version of a statistical software. Statistical analysis and modeling of the response variable, namely the degradation rate constant (KC), were carried out using analysis of variance (ANOVA) with a 95% confidence level, Pareto charts, and response surface modeling (RSM).

RSM assessed potential relationships between experimental factors and response variables based on selected criteria and determined optimal operational conditions. Further details regarding this methodology can be found elsewhere [19,35]. The parameter ranges were established by considering preliminary experimental results and information from the literature [19,36] resulting in a time of electrolysis ranging from 30 to 90 min, initial ACTP concentration from 10 to 30 mg L⁻¹, and applied current density from 20 to 60 mA cm⁻². Other variables were maintained at constant conditions, including supporting electrolyte concentration, volume, pH, and Fe²⁺ concentration.

RSM involved adjusting experimental results to a second-order multivariable polynomial model for analysis:

$$Y_i={\beta }_0+{\sum }_{i=1}^n{\beta }_i{x}_i+{\sum }_{i=1}^n{\beta }_{ii}{x}_i^2+{\sum }_{j=i+1}^n{\beta }_{ij}{x}_i{x}_j$$

(6)

where β₀, β_i, β_ii, and β_ij are the regression coefficients for the intercept, linear, square, and interaction terms, respectively; Y_i is the response variable, and x_i and x_j are independent variables [24,27,29,32,36]. The models’ quality and prediction capacity were evaluated based on the variation coefficient of determination (R²).

3. Results and Discussion

3.1. Effect of Applied Current Density

The current density (j) proved to be one of the most influential parameters affecting process efficiency and the kinetic degradation constant. An increase in j led to an enhanced generation rate of oxidizing radicals (such as ^•OH), both at the surface of the BDD anode (adsorbed M(^•OH) species) and in the bulk solution, due to Fenton-type reactions in the presence of Fe²⁺ and cathodic generation of H₂O₂ in EF/SPEF processes. In our direct EO experiments using BDD/Graphite electrodes (without Fe²⁺ addition), increasing j from 20 to 60 mA cm⁻² significantly accelerated the removal of 10 mg L⁻¹ ACTP. For instance, at j = 60 mA cm⁻², ~92% degradation was achieved in 50 min, compared to only ~80% at j = 20 mA cm⁻², and in the SPEF process, we obtain a 90% and 99% degradation in 50 min with 20 and 60 mA cm⁻², respectively (see Figure 2). Accordingly, the pseudo-first-order rate constant ranged from ~0.0360 min⁻¹ (at 20 mA cm⁻²) to ~0.0545 min⁻¹ (at 60 mA cm⁻²) in the EO process and 0.0526 (at 20 mA cm⁻²) to 0.0835 (at 60 mA cm⁻²) for the SPEF process. This notable enhancement can be attributed to the increased formation of oxidizing species with higher current density.

Furthermore, operating at high current densities in diamond electrodes under acidic sulfate media may promote additional oxidative pathways. Our results suggest that at elevated j values, BDD not only generates more hydroxyl radicals but may also facilitate the formation of secondary oxidizing species, such as sulfate radicals. Previous studies have proposed that stable anodes, such as BDD, can oxidize sulfate ions (SO₄²⁻) to peroxydisulfate (S₂O₈²⁻) at sufficiently high potentials. Although weaker than ^•OH, the S₂O₈²⁻ species can still contribute to the homogeneous degradation of organic pollutants in solution. The possible generation of peroxydisulfate in our EO trials with 0.5 M Na₂SO₄ at pH 3.0 could partially explain the enhanced removal of ACTP at higher current densities, in agreement with the prior literature reporting S₂O₈²⁻ formation mediated by hydroxyl radicals at non-active anodes.

3.2. Effect of Initial ACTP Concentration

The initial concentration of the pollutant (C₀) is another critical factor influencing degradation performance. Higher degradation percentages and slightly improved kinetic constants were obtained when treating more diluted ACTP solutions, while higher initial concentrations tended to slow down the relative removal rate. For instance, in electro-Fenton (EF) experiments (BDD/BDD, j = 20 mA cm⁻², pH 3.0), the degradation percentages after 60 min were approximately 86.2%, 92.8%, and 97.5% for initial ACTP concentrations of 10, 20, and 30 mg L⁻¹, respectively (Figure 3). Accordingly, the apparent pseudo-first-order rate constant decreased as C₀ increased, with values of approximately 0.0619, 0.044, and 0.033 min⁻¹, respectively.

This inverse relationship can be attributed to several factors:

(i) At higher concentrations, the same absolute amount of hydroxyl radicals (^•OH) results in a lower fractional removal of ACTP, as predicted by pseudo-first-order kinetics.

(ii) More ACTP molecules generate more intermediate byproducts, some of which can act as radical scavengers, competing with the ACTP compound for oxidizing species. Indeed, phenolic intermediates can react rapidly with hydroxyl radicals, reducing the availability of oxidants.

3.3. Effect of Electrolysis Time

Electrolysis time naturally influences the extent of pollutant removal: longer treatment periods allow for higher degradation levels, although the marginal returns tend to decrease over time. In our experiments, treatment times typically extended to 60 or 90 min. Most of ACTP was removed within 30–45 min, particularly during electro-Fenton (EF) and SPEF processes. Extending the reaction time from 60 to 90 min provided additional benefits, mainly in enhanced mineralization (i.e., COD removal).

From a kinetic standpoint, an interesting effect was observed in both EO and EF processes: the apparent pseudo-first-order rate constant calculated using data up to 60 min was slightly higher than that calculated using the full 90 min dataset. This suggests a decrease in the average reaction rate over longer intervals, possibly due to the accumulation of intermediate species that scavenge oxidants or a decline in system reactivity. For example, in EF, the effective concentration of Fe²⁺ might gradually decrease due to slow precipitation or other side reactions.

In contrast, no such decline was observed in the SPEF process. The COD value at 60 min was nearly identical to that at 90 min (≈0.0395 min⁻¹), indicating that the continuous irradiation sustains oxidant production and prevents passivation effects even at later stages of treatment. These results suggest that for SPEF, 60 min is typically sufficient to remove most of the ACTP, and additional treatment time primarily targets persistent residuals affecting COD rather than significantly increasing the degradation rate.

On the other hand, for EF and EO, extending the treatment duration from 60 to 90 min did improve final removal percentages, although with reduced efficiency, as evidenced by the slight decrease in the sign that degradation reactions slow down as the process progresses.

From a process optimization perspective, minimizing the required electrolysis time to meet a target removal efficiency is desirable for energy savings and to preserve electrode longevity. Boron-doped diamond (BDD) electrodes have a finite lifespan, which depends on the operational conditions. Prior studies by our group [31,37] estimated a median lifetime of approximately 782 h for BDD electrodes operated at current densities between 10 and 50 mA cm⁻². Thus, a single 90 min experiment conducted at 60 mA cm⁻² consumes less than 0.2% of the estimated electrode lifespan, suggesting that the treatment durations employed in this study (≤90 min) are safe and do not significantly compromise electrode integrity.

3.4. Reaction Order and Kinetic Constants

The ACTP concentration profiles as a function of time, determined by HPLC, were fitted to kinetic models of various orders. In all cases, the degradation of ACTP followed a pseudo-first-order kinetic model concerning its concentration. This was evidenced by the linear behavior of the plots versus time (Figure 4), which yielded correlation coefficients (R²) typically between 0.97 and 0.99.

The apparent kinetic constants varied depending on the operational parameters (electrochemical process, current density, initial concentration, etc.) and ranged from approximately 0.036 to 0.084 min⁻¹. Specifically, for the electrochemical degradation of ACTP (30 mg L⁻¹, pH 3.0, 0.5 M Na₂SO₄) at different current densities (20–60 mA cm⁻²), values between 0.036 and 0.0545 min⁻¹ were obtained. This broad range highlights the influence of process variables on reaction rate.

The pseudo-first-order behavior suggests that, under our experimental conditions, the concentration of oxidizing radicals (e.g., ^•OH) remains approximately constant during most of the treatment period, indicating a mass-transport-controlled regime. Consequently, the degradation rate of ACTP is proportional to its residual concentration throughout this phase of the reaction.

3.5. Comparison Between EO, EF, and SPEF Processes

The results establish a clear efficiency hierarchy among the studied electrochemical processes: SPEF > EF > EO. This trend was consistently observed across reaction rates (kinetic constants), ACTP removal percentages, and COD reduction.

For instance, under identical operating conditions (pH 3.0, 30 mg L⁻¹ ACTP, j = 20 mA cm⁻², 60 min), the SPEF process achieved up to ~97% ACTP removal in 60 min, compared to ~90.6% with EF and ~82.9% with EO. The corresponding kinetic constants were 0.084 min⁻¹ (SPEF), 0.0619 min⁻¹ (EF), and 0.045 min⁻¹ (EO), indicating that the oxidation rate in SPEF was nearly twice that of EF and almost three times that of EO. Similarly, COD removal was higher with SPEF: ~78% mineralization was achieved in 60 min under SPEF conditions, compared to typical values of 50–60% for EF and <50% for EO under similar starting conditions (based on our COD data and the extrapolated literature values).

The superior performance of SPEF stems from the synergistic action of multiple oxidative mechanisms operating simultaneously:

(i) Direct anodic oxidation at the BDD electrode, which produces highly reactive adsorbed hydroxyl radicals (BDD(^•OH)) at the electrode surface (heterogeneous oxidation);

(ii) Electro-Fenton oxidation, in which dissolved oxygen is electrochemically reduced at the cathode (graphite or BDD) to generate H₂O₂ in situ, which in the presence of catalytic Fe²⁺ yields homogeneous ^•OH radicals in solution;

(iii) UV/solar photoactivation enhances degradation through multiple pathways: direct photolysis of conjugated intermediates, photocatalytic degradation of Fe³-organic complexes, and photoreduction of Fe³⁺ to regenerate Fe²⁺, maintaining the Fenton cycle via light-assisted reactions.

In our SPEF trials, the inclusion of solar UV radiation markedly accelerated the degradation rate of ACTP compared to equivalent systems without irradiation. Solar light promoted the photoreduction of Fe³⁺ to Fe²⁺ (via Fe(OH)²⁺ and aqueous Fe³⁺ complexes), maintaining the catalytic cycle and preventing Fe(III) accumulation. Additionally, solar irradiation facilitated the photodegradation of aromatic intermediates, such as quinones, which absorb UV and can be photoconverted to more easily oxidizable carboxylic acids. As a net effect, SPEF led to faster ACTP removal, improved mineralization, and reduced accumulation of colored intermediates compared to EF or EO.

These findings align with previous studies showing that photo-Fenton (solar or UV) outperforms dark Fenton and simple electrochemical oxidation. For example, ref. [33] photo-Fenton was the most effective among various AOPs tested for ACTP degradation, consistently achieving >90% removal where other processes fell short.

Regarding the comparison between EF and EO, our results demonstrate the advantage of incorporating the Fenton system into the electrochemical treatment. EF outperformed EO in both degradation and mineralization. This is attributed to the dual-pathway oxidation in EF, which involves heterogeneous anodic oxidation at BDD (similar to EO) and homogeneous oxidation via radicals generated within the bulk solution. In direct EO, degradation relies exclusively on surface-generated hydroxyl radicals and secondary electrochemical oxidants (e.g., persulfates). While BDD is a powerful electrode material, mass transport of pollutants to the surface may limit the overall rate (diffusion control), and some intermediates may not be effectively oxidized solely at the interface.

Conversely, EF enables the generation of ^•OH throughout the solution volume (via H₂O₂ + Fe²⁺), allowing for efficient bulk-phase degradation of parent compounds and soluble intermediates that may not reach the anode. As a result, EF demonstrated higher reaction rates and more complete mineralization than EO under identical conditions, as reflected by our measurements (%COD SPEF, %COD EF > %COD EO). The literature consistently supports this trend. For instance, ref. [38] demonstrated that anodic oxidation alone using a Ti₄O₇ electrode achieved complete ACTP degradation but resulted in lower mineralization (~19% TOC removal), highlighting the persistence of oxidation-resistant byproducts. In contrast, the integration of electro-Fenton conditions significantly enhanced both degradation kinetics and mineralization yield (~44%), confirming that combining homogeneous and heterogeneous oxidation pathways is more effective in addressing stable intermediates such as p-benzoquinone and short-chain carboxylic acids.

3.6. HPLC Analysis

A high-performance liquid chromatography (HPLC) method with UV detection was developed and optimized to enable more specific monitoring of ACTP and the formation of intermediate byproducts. HPLC analysis of a standard mixture containing ACTP, catechol (CAT), and phenol (PHE), each at 15 mg L⁻¹, was performed, obtaining well-defined and non-overlapping peaks.

In untreated samples, only the ACTP peak was observed. In contrast, treated samples, particularly those subjected to the more aggressive SPEF processes, showed an almost complete disappearance of the ACTP peak, indicating high conversion efficiency. Simultaneously, the transient formation of phenol and catechol was detected in chromatograms of samples collected at intermediate reaction times (Figure 5). These intermediates appeared in moderate concentrations, reached a relative maximum during mid-treatment, and then declined, suggesting they were further oxidized by the advanced oxidation processes (AOPs). This behavior indicates that phenol and catechol act as degradable intermediates: formed from ACTP, temporarily accumulated, and ultimately removed upon prolonged treatment, especially under the more oxidative SPEF conditions involving solar UV radiation.

The identification of catechol and phenol agrees with the literature on ACTP degradation. Previous studies on advanced oxidation have reported the formation of hydroxylated aromatic compounds as key intermediates. For example, Periyasamy group [39] reported that electro-oxidation of ACTP using graphite anodes primarily yields hydroquinone (1,4-dihydroxybenzene) and benzoquinone, while other studies have also detected 4-aminophenol and aniline in partially treated water. In our case, the HPLC method was sensitive to phenol and catechol. However, it did not directly detect 4-aminophenol or other derivatives, likely due to the detection wavelengths employed (243 and 475 nm) or because these species were present at very low or transient concentrations.

Nevertheless, the detection of phenol and catechol provides clear evidence of hydroxylation and ring-opening reactions occurring on the aromatic structure of ACTP during treatment. These simpler aromatic species were also eventually degraded, consistent with the significant decrease and eventual disappearance of their HPLC signals at longer electrolysis times (e.g., after 90 min, phenol and catechol peaks were no longer detectable, particularly under SPEF conditions).

3.7. COD Analysis

In general, COD removal was lower than ACTP concentration removal, which was expected due to the formation of refractory organic byproducts during the oxidative process. Under the most oxidative conditions, specifically, in SPEF treatments at j = 60 mA cm⁻², [Fe²⁺] = 0.05 mM, and pH 3.0, mineralization was significant but incomplete. Figure 6 shows that up to ~63.5% of COD was removed after 60 min of a 10 mg L⁻¹ ACTP solution treatment. This indicates that approximately three-quarters of the total organic carbon was converted into CO₂ within one hour of SPEF treatment, with the remaining ~22% persisting in solution as resistant organic intermediates.

These findings are consistent with those reported by other researchers. For instance, [40] observed this while working at pH 2.6 with a magnetite catalyst. Total degradation was found in the assays with 500 mL of 100 mg L⁻¹ ACTP with 6 g L⁻¹ catalyst at 60 °C after a long treatment time of 300 min, but TOC was only reduced by 50%. This supports the idea that the complete mineralization of ACTP requires extended treatment times or more aggressive oxidative conditions due to the persistence of relatively stable byproducts.

From the COD decay data in SPEF, mineralization kinetics were fitted to a pseudo-first-order model, as evidenced by the linear plot versus time, with a correlation coefficient of R² ≈ 0.973. The corresponding apparent rate constant was ≈0.017 min⁻¹. This value is of the same order of magnitude as the degradation constants for ACTP determined by HPLC, though slightly lower, reflecting the slower oxidation rate of intermediate compounds. The pseudo-first-order behavior for COD suggests that overall mineralization is limited by the diffusion of organic molecules to the anodic surface and reaction zone, as commonly observed in electrochemical oxidation using non-active electrodes.

Additionally, based on COD data, the current efficiency index (ICE), specific energy consumption (SEC), and energy oxidation index (EOI) were calculated (Figure 6). Initially, the ICE was relatively high, reaching 5.97 at the beginning of the reaction. This value gradually decreased as the more readily oxidizable compounds were consumed, reaching ~3.2 after 90 min of treatment. The current efficiency decline is consistent with the accumulation of less reactive intermediates and the decrease in substrate concentration, which lowers the fraction of current effectively used in organic oxidation while increasing losses due to parasitic reactions such as oxygen evolution.

Nevertheless, an ICE of 3.2 after 90 min indicates a reasonably good energy utilization toward mineralization. Regarding energy consumption, the process required approximately 0.052 kWh per gram of COD removed after 90 min, which is relatively low for an advanced electrochemical oxidation process. This result highlights the efficiency of the SPEF process, which demonstrated strong oxidative power at a relatively low energy cost compared to alternative treatments.

Comparable mineralization efficiencies have been reported in the literature. For example, in a pilot-scale SPEF system (10 L, 157 mg L⁻¹ ACTP, Pt anode), a current efficiency of ~71% and TOC reduction in ~75% were achieved after 150 min, values that align well with our results when accounting for differences in scale and initial concentration [41].

4. Identification of Transformation Products and Proposed Degradation Mechanism

Based on experimental evidence, including HPLC identification of intermediates, voltametric observations, and COD/color changes, a tentative mechanism can be proposed for the advanced electrochemical degradation of ACTP using BDD/Fe²⁺ electrodes under solar irradiation (Figure 7). In general, ACTP undergoes initial oxidative attacks at both the aromatic ring and the amide group, leading to ring cleavage and the formation of lower-molecular-weight compounds, ultimately resulting in mineralization into CO₂, H₂O, and oxidized nitrogen species (primarily nitrates).

Step 1—Amide cleavage and formation of 4-aminophenol:

A plausible primary pathway is the oxidative cleavage of the amide N–C bond by hydroxyl radicals, either at the nitrogen or the adjacent carbon atom. This leads to the formation of 4-aminophenol (2) (PAP) and acetic acid. Although PAP (2) was not directly detected by our HPLC setup (possibly due to its rapid conversion or detector limitations), it has been reported in Fenton-based ACTP degradation, often alongside aniline. Therefore, we assume that part of the ACTP is transiently converted to PAP (2).

Step 2—Aromatic hydroxylation and formation of hydroquinone/catechol:

^•OH radicals further hydroxylate the ACTP (1) (or PAP (2)) aromatic ring. Two main routes are possible:

(a) Para-hydroxylation leads to hydroquinone (1,4-dihydroxybenzene) (6) if the amino group is replaced by –OH.

(b) Ortho-hydroxylation yields catechol (1,2-dihydroxybenzene) (4), either from phenol or via transformation of 2-aminophenol (3) (from PAP).

Our HPLC data confirmed the presence of catechol (4) and phenol (7), suggesting that both aromatic deamination and hydroxylation occurred. The presence of phenol (7) implies the complete loss of the para-substituent (–NHCOCH₃ or –NH₂), possibly through formation and subsequent oxidation of aniline. Aniline yields phenol (7) through oxidative deamination (–NH₂ → –OH + NH₄⁺). Catechol (4) is likely formed via ortho-hydroxylation of phenol (7), whereas hydroquinone (6) may arise from para-hydroxylation. Although hydroquinone (6) was not explicitly detected in our chromatograms, it may have rapidly converted to benzoquinone (5) or coeluted with catechol (4), preventing its precise identification.

Step 3—Ring-opening and formation of organic acids:

Once phenolic derivatives (catechol (4), hydroquinone (6)) or p-benzoquinone (5) form, the benzene ring becomes more susceptible to further oxidative cleavage. ^•OH radicals can open the ring, forming oxygenated open-chain compounds, typically short-chain dicarboxylic acids such as oxalic (11), maleic (10), fumaric (9), acetic (12), and formic acids (8). While we did not specifically identify these acids by HPLC (which would require ion chromatography or LC-MS), the significant COD removal observed, especially in SPEF, suggests their formation. These acids form in ACTP degradation and can chelate Fe³⁺, inhibiting Fenton reactions if they accumulate. However, UV irradiation in SPEF promotes the photodegradation of Fe³⁺-carboxylate complexes, regenerating Fe²⁺ and enabling continued oxidation. This contributes to the higher mineralization observed in SPEF than in EF or EO.

We postulate that catechol (4) and hydroquinone (6) are sequentially oxidized to ortho and p-benzoquinone (5), respectively, then to maleic (10)/muconic (13) acid derivatives, and, finally, to oxalic (11), formic (8), and acetic acids (12), eventually yielding CO₂ and H₂O. This cascade of reactions is consistent with the ~78% COD removal observed in SPEF and the previous literature reports, indicating 70–80% mineralization and nitrogen predominantly converted to nitrate (NO₃⁻).

Fate of nitrogen:

The nitrogen atom in ACTP and its derivatives (PAP, aniline) can undergo various oxidative transformations. Under strongly oxidative aerobic conditions, conversion to nitrate (NO₃⁻) is typically favored. Although nitrates were not quantified in our study, previous work has shown 50–100% N to NO₃⁻ conversion, depending on the process. The oxidative strength of BDD and high ^•OH flux strongly support this pathway. A minor fraction may have formed NH₄⁺ via incomplete oxidation, but this, too, can be further oxidized to NO₃⁻ under excess ^•OH. Volatilization of nitrogen as NO_x gases was unlikely, as no odor or visible release was noted. Thus, it is reasonable to assume that most nitrogen was mineralized to aqueous nitrate.

This proposed mechanism aligns well with the experimental findings. The initial formation of 4-aminophenol and/or aniline explains the need for sufficient oxidant, as these species can slow degradation. The confirmed presence of catechol (4) and phenol (7) validates the aromatic degradation pathways. Previous HPLC-MS studies have reported up to seven aromatic intermediates, including hydroxylated compounds and quinones, supporting the complex degradation pathway described here. Each oxidation step increases polarity and decreases molecular weight, consistent with the staged COD decay observed in our experiments; quick at early times, and slower at later stages due to increasingly recalcitrant intermediates.

5. Optimization via Experimental Design

Results of the regression analysis were presented for MMO and BDD anodes, providing expressions for the discoloration rate degradation of ACTP.

The Box–Behnken design results for SPEF showed K_C values ranging from ~0.016 to 0.084 min⁻¹ depending on the factors tested. ANOVA analysis revealed that initial concentration and current density were the most influential variables, followed by some second-order interactions. At the same time, electrolysis time had a less pronounced effect within the studied range. At least five terms were found to be statistically significant (p < 0.05) concerning K_C.

The fitted quadratic model explained ~97.47% of the variability in the kinetic constant (R² = 0.9747), with an adjusted R² = 0.9409, indicating excellent fit and minimal overfitting. This confirms that a response surface methodology (RSM) approach is well-suited to describe the behavior of K_C as a function of operational parameters in the SPEF process.

The response surface plots (Figure 8 and Figure 9) revealed two main trends:

(1) Higher current densities and lower initial concentrations resulted in higher kinetic constants, with surfaces ascending toward the high-j, low-C₀ corner.

(2) The effect of electrolysis time was neither linear nor monotonic: increasing the duration from 30 to ~60 min enhanced K_C, but beyond ~60–75 min, the benefit plateaued or slightly declined, at least in the absence of light. However, under continuous solar irradiation (as in SPEF), time ceased to be a limiting factor once a threshold was reached. This is consistent with our experimental observations, which showed that 60 min was generally sufficient to maximize the reaction rate in SPEF, and longer times did not significantly increase K_C.

Pareto analysis confirmed that quadratic terms (e.g., C₀²) and interaction terms such as concentration × current (BC) also played significant roles (Figure 10). These effects reflect the curvature of the response surface, possibly due to saturation effects at very high current densities or very low initial concentrations.

By applying the desirability function to maximize K_C, an optimal parameter set was identified with a desirability value close to 1 (0.9998). The optimal conditions corresponded to approximately 89–90 min, 10 mg L⁻¹ of ACTP, and 60 mA cm⁻², with a predicted kinetic constant of COD ≈ 0.062 min⁻¹. Notably, while the model suggests an optimum slightly above 60 min, the incremental increase in K_C beyond 60 min is marginal (from 0.058 to 0.072 min⁻¹). Thus, 60 min can be practically chosen as the treatment time in SPEF to save energy while accepting a negligible reduction in K_C. The optimal prediction was validated experimentally, yielding K_C values close to the model predictions and within the 95% confidence interval.

6. Conclusions

This study confirms that the solar photo-electro-Fenton (SPEF) process is a highly efficient and sustainable electrochemical technology for the removal of acetaminophen (ACTP) from aqueous media. Compared to electro-oxidation (EO) and electro-Fenton (EF), SPEF exhibited superior degradation and mineralization performance, achieving up to 97% ACTP removal and 78% COD reduction within 90 min under optimized operational conditions (current density = 60 mA cm⁻², [Fe²⁺] = 0.05 mM, pH 3.0). Kinetic analysis revealed pseudo-first-order behavior, and the Box–Behnken design highlighted the initial pollutant concentration and current density as key process parameters.

The identification of phenol and catechol as primary intermediates via HPLC analysis supported a degradation pathway involving hydroxylation, aromatic ring cleavage, and progressive oxidation to low-molecular-weight acids. Importantly, the use of natural solar irradiation in SPEF enhanced oxidant regeneration and intermediate breakdown, significantly improving energy efficiency (~0.052 kWh gCOD⁻¹).

These findings demonstrate the potential of SPEF as a cost-effective, environmentally friendly alternative for the treatment of pharmaceutical contaminants, especially in decentralized or resource-constrained regions. The integration of solar energy into electrochemical advanced oxidation not only improves process performance but also aligns with sustainable engineering principles, offering a scalable solution for the remediation of emerging pollutants in water treatment applications.