A Sustainable Aluminium-Based Electro-Fenton Process for Pharmaceutical Wastewater Treatment: Optimization, Kinetics, and Cost–Benefit Analysis

Abstract

Pharmaceutical contamination poses growing environmental risks, yet industrial adoption of advanced oxidation processes (AOPs) remains limited by high costs and the environmental impacts associated with specialized electrodes. This study demonstrates that unmodified aluminum electrodes achieve pharmaceutical degradation performance comparable to precious metal systems at dramatically reduced cost and carbon footprint. An aluminum-based electro-Fenton (EF) system was optimized for amlodipine (AML) removal through systematic evaluation of operational parameters. Under optimized conditions (pH 2.7, 35 mg L⁻¹ FeCl₃, 1.3 mM NaCl, 5 V), the system achieved 97% AML degradation within 15 min, following pseudo-first-order kinetics ($k=0.15$ min⁻¹). The mechanism combines hydroxyl radical oxidation with synergistic electrocoagulation resulting from anodic Al³⁺ release and cathodic Fe²⁺ regeneration. Sustainability assessment revealed exceptional performance: an energy consumption of 0.32 kWh m⁻³, a carbon footprint of 0.53 kg CO₂-eq m⁻³ (60–75% lower than conventional AOPs), and operational costs of $0.71–1.05 m⁻³. Aluminum electrodes cost 100× less than platinum alternatives, with the generated Al(OH)₃ sludge offering valorization potential. This work demonstrates that high-performance electrochemical remediation is achievable using Earth-abundant materials, providing a scalable and cost-effective alternative for pharmaceutical wastewater treatment in resource-constrained settings.

1. Introduction

Water pollution resulting from human activities poses a significant global threat. The ongoing discharge of both organic and inorganic substances into water bodies leads to a decline in water quality, affecting its suitability for consumption and the survival of various organisms. Daily consumption of numerous industrial chemicals, herbicides, dyes, and pharmaceuticals released into natural waters are contributing to this issue [1]. This pollution is exceptionally stable under the influence of ambient conditions such as sunlight and environmental temperature, which promote its accumulation in various water matrices (e.g., surface waters and drinking water), generally at concentrations ranging from mg L⁻¹ to ng L⁻¹ [2]. Most of these compounds are toxic and can cause serious health problems in living organisms, and even low concentrations of certain drugs are capable of causing serious problems [1,3].

Among the several types of pollutants in aquatic systems, pharmaceuticals are one primary class of contaminants of concern. These chemical agents are used in both human and veterinary applications and include a wide range of therapeutic molecules, such as antibiotics, antihypertensives, analgesics, and hormones. Pharmaceutical residues may dissolve and persist in the aqueous phase or adsorb onto suspended solids and sediments, depending on their hydrophilicity, molecular weight, and charge.

Pharmaceuticals have been detected across multiple environmental matrices worldwide, including surface waters, groundwater, wastewater, and even drinking water sources. This is mainly due to incomplete removal in conventional wastewater treatment plants, which are not designed to efficiently degrade these micropollutants efficiently. Domestic discharges, effluents from hospitals and pharmaceutical industries, leachates from landfill sites, and run-off from intensive livestock farming operations are considered the principal sources of these contaminants [4].

Given these facts, human and veterinary pharmaceuticals are considered significant contributors to the chemical pollution of aquatic environments, possibly posing ecological and health risks through bioaccumulation, disruption of microbial communities, and the promotion of antimicrobial resistance [4,5].

Indeed, the detected concentrations of drugs in surface waters are generally on the order of mg L⁻¹ or ng L⁻¹. However, the concentrations measured at the discharge point of some industries, especially pharmaceutical industries, can reach levels of around mg L⁻¹ [6]. These contaminations are well documented, with hormones, anti-inflammatories, antidepressants, antibiotics, and many other drugs found in small creeks, lakes, rivers, drinking water, marine environments, and occasionally in groundwater [7]. Groundwater contamination mainly occurs through the infiltration of surface water containing pharmaceutical residues and leaks from landfill sites and sewer systems.

Conventional wastewater treatment is often ineffective at removing persistent organic pollutants, and sewage treatment plants (STPs) [5] are recognized as the primary pathway by which human pharmaceuticals enter aquatic environments. The discharge of pharmaceuticals as STP effluents into rivers and lakes is becoming a growing concern [4,8].

Among everyday pharmaceutical products, the compound selected for this study is amlodipine (AML), a long-acting calcium channel blocker widely used as an antihypertensive, for angina, and for decompensated cardiac failure [9]. It helps lower blood pressure and treat angina. It is a third-generation, long-acting dihydropyridine calcium channel blocker [10].

Various conventional technologies have been employed to remove pharmaceuticals from wastewater, including inefficient liquid-to-solid-phase mass transfer methods such as adsorption on activated carbon, coagulation, flocculation, precipitation, and filtration. These processes generate secondary sludge that requires complex decontamination treatment. To address the limitations of conventional wastewater treatment methods, research over the past 30 years has developed highly effective techniques for the total removal of persistent organic pollutants from wastewater and surface water. The relevance of advanced oxidation processes (AOPs) stems from their ability to generate hydroxyl radicals (·OH) in situ. These radicals exhibit high reactivity and non-selectivity, enabling the degradation of a broad spectrum of organic compounds, including recalcitrant ones [1,2]. Currently, numerous AOP technologies are under development for the removal of organic contaminants from wastewater, including chemical, electrochemical, photochemical, and photoelectrochemical approaches. Among these, photoelectrochemical systems combining semiconductor materials with noble metal nanoparticles have shown enhanced degradation efficiency for pharmaceutical compounds under solar irradiation [2,11,12].

The Fenton process has been among the first proposed and Omnipresent ·OH-based AOPs using an H₂O₂/Fe²⁺ mixture. The Fenton reaction rapidly destroys organic pollutants at optimal conditions, limited to acidic pH, generating highly reactive hydroxyl radicals that effectively break down even persistent contaminants into smaller, less harmful byproducts, with a large sludge production of Fe(III) hydroxide that requires further, expensive treatment. More potent Fenton-based processes, such as photo-Fenton and electro-Fenton, have been developed to produce very little or no sludge and to be applied to even near-neutral media [13]. Recent research on the removal of pharmaceuticals from wastewater by AOPs has reported complete degradation in some cases, although only electro-Fenton treatment achieved total mineralization. Photo-Fenton is also one of the AOPs for which solar technologies have been most extensively studied and developed, as it is low-cost, easy to handle, and well adapted to small- and medium-scale renewable energy facilities. Recent developments in surface-modified catalysts have demonstrated improved H₂O₂ activation at low concentrations through enhanced electron transfer mechanisms, addressing some of the limitations associated with high peroxide requirements [14]. However, there are still a few challenges associated with conventional treatments, including storage, byproduct formation, high catalyst consumption, and the need for highly concentrated H₂O₂ [15,16]. Therefore, efforts have been undertaken to develop new technologies to improve the oxidation efficiency of the Fenton process [17,18].

Among electrochemical AOPs, the electro-Fenton (EF) process has emerged as an up-and-coming technology due to its ability to generate H₂O₂ in situ via cathodic oxygen reduction, thereby eliminating the safety hazards and logistical challenges associated with transporting and storing concentrated H₂O₂ [19,20]. However, widespread industrial implementation of EF systems has been constrained by the high capital costs of specialized electrode materials, particularly boron doped diamond (BDD) anodes ($15,000–25,000/kg) and platinum cathodes ($50,000–80,000/kg), which account for 40–60% of total system costs in commercial installations [14,16,21]. Additionally, the energy intensity of high-voltage BDD systems (typically 10–30 V, consuming 2–5 kWh/m³) and the environmental burden of noble metal extraction (15,000 kg CO₂-eq/kg for platinum production) raise questions about the true sustainability of these advanced technologies [12,21,22].

Electrochemical advanced oxidation processes (EAOPs), also called anodic oxidation, are a new class of advanced oxidation processes based on electrochemical technology and among the most widely used for water remediation; they involve the oxidation of a contaminated solution in a divided or undivided electrochemical cell, usually operating at constant current density. The hydroxyl radical (·OH) interacts indiscriminately with a wide range of pollutants, both organic and inorganic. Electro-Fenton, one of the enhanced versions of the Fenton process, has been developed, and it is attracting considerable interest for the removal of organic pollutants. In an electro-Fenton process, pollutants are removed using Fenton reagents and anodic oxidation on the anode surface. Anodic oxidation alone is not effective for mineralizing most aromatic pollutants, due to the formation of resistant carboxylic acids [12,23]. However, owing to the production of ·OH, the electro-Fenton process can achieve significant efficiency in the mineralization of organic pollutants. Unlike the traditional Fenton process, electro-Fenton produces H₂O₂ on-site, avoiding the need to transport and store external H₂O₂; therefore, it is an environmentally friendly technology, since it reduces the use of chemicals [20,24].

The two-electron cathodic reduction of oxygen (O₂) is a recognized pathway for hydrogen peroxide (H₂O₂) production. In an acidic medium, this reaction (Reaction (1)) can occur with oxygen introduced either directly into the solution or onto the cathode surface, with E⁰ = +0.68 V/SHE [25]. This reaction competes with the cathodic reduction of H⁺ to H₂, with an E⁰ = +0 V/SHE, and is more readily achieved than the four-electron cathodic reduction of O₂ to H₂O, with an E⁰ = +1.23 V/SHE. The cathode shows higher electrocatalytic activity than reaction (1), making it ideally suited to H₂O₂ production under homogeneous conditions. However, the rate of H₂O₂ accumulation in the electrolyte solution depends on the type of cathode material used and the operating conditions [21]. When an undivided cell is used, this species is oxidized to O₂ at the anode surface via the hydroperoxyl radical (HO₂•) in reaction (2) [25]. This reaction inhibits the anodic oxidation of water to O₂, giving rise to the heterogeneous intermediate ·OH (formed by reaction (3)) that is limited to the anode surface [1,2].

$$\begin{array}{cc}\hfill {\mathrm{O}}_2+2{\mathrm{H}}^{+}+2e^{-}& \to {\mathrm{H}}_2{\mathrm{O}}_2\hfill \end{array}$$

(1)

$$\begin{array}{cc}\hfill {\mathrm{H}}_2{\mathrm{O}}_2& \to {\mathrm{HO}}_2•+{\mathrm{H}}^{+}+e^{-}\hfill \end{array}$$

(2)

$$\begin{array}{cc}\hfill \mathrm{M}+{\mathrm{H}}_2\mathrm{O}& \to \mathrm{M}(\mathrm{OH}•)+{\mathrm{H}}^{+}+e^{-}\hfill \end{array}$$

(3)

$$\begin{array}{cc}\hfill 3{\mathrm{H}}_2\mathrm{O}& \to {\mathrm{O}}_3+6{\mathrm{H}}^{+}+6e^{-}\hfill \end{array}$$

(4)

$$\begin{array}{cc}\hfill {\mathrm{Fe}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2& \to {\mathrm{Fe}}^{3+}+\mathrm{OH}•+{\mathrm{OH}}^{-1}{k}_2=63{\mathrm{M}}^{-1}{\mathrm{s}}^{-1}\hfill \end{array}$$

(5)

Reaction (3) generates the primary oxidizing agent M(·OH) of organic pollutants in AO. This process can also be carried out via cathodic generation of H₂O₂, which has slightly higher oxidation power than AO due to H₂O₂’s very low oxidation capacity [1]. Although H₂O₂ treatment alone is not practically relevant, it serves as an essential blank experiment. Its purpose is to benchmark the oxidative action of homogeneous ·OH from the Fenton reaction against the heterogeneous M(·OH) produced at the anode in Fenton-based EAOPs. The anode material also plays a significant role in the Electro-Fenton process [26,27,28,29].

To address these economic and environmental barriers, this study demonstrates, for the first time, that unmodified aluminum electrodes can serve as both anode and cathode in a unified EF reactor for the degradation of pharmaceutical pollutants, achieving performance comparable to precious-metal systems at a fraction of the cost and environmental impact. While prior research has explored aluminum electrodes in coupled electrocoagulation–EF configurations for antibiotic treatment [10], their systematic optimization for pharmaceutical degradation and comprehensive sustainability assessment remain unreported. Our aluminum-based configuration uniquely integrates three synergistic mechanisms within a single reactor: (i) cathodic H₂O₂ generation via oxygen reduction, (ii) anodic Al³⁺ release for electrocoagulation, and (iii) continuous Fe²⁺ regeneration at the cathode. This integrated design reduces system complexity by eliminating the need for separate coagulation units, lowers operational costs by 60–75% compared to BDD-based systems, and achieves a carbon footprint of 0.53 kg CO₂-eq m⁻³, 60–75% lower than conventional Fenton and ozonation processes.

This work provides comprehensive parametric optimization (pH, AML concentration, FeCl₃ and H₂O₂ dosing, electrolyte type) and kinetic characterization of amlodipine degradation, coupled with rigorous energy-efficiency and life-cycle carbon footprint analyses. The results demonstrate that aluminum-based EF can achieve 97% pharmaceutical removal within 15 min while consuming only 0.32 kWh m⁻³, representing a scalable, economically viable, and environmentally responsible pathway for treating pharmaceutical-contaminated wastewater.

2. Materials and Methods

2.1. Reagents

Chemicals and Reagents: All chemicals used in this study were of analytical grade. The analytical standard of AML was purchased from Sigma-Aldrich. Hydrogen peroxide (H₂O₂) and iron chloride (FeCl₃) were obtained from Biochem. The solution’s pH was adjusted using sulfuric acid (H₂SO₄) and sodium hydroxide (NaOH) (Biochem). Sodium sulfate (Na₂SO₄) and sodium chloride (NaCl) were purchased from Biochem. An individual stock solution of AML at the concentration level of 0.1 g/L was prepared using ultrapure water and stored at 23 °C. Ultra-pure water was used to prepare solutions with high-purity; it was obtained from an Adrona_E30 water purification system. All chemical substances were used as received from the supplier, without modification.

2.2. Experimental Procedure

The Fenton and the electro-Fenton experiments were performed in a batch reactor with a surface area of ≈$50.27{\mathrm{cm}}^2$, open to the atmosphere. The reactor was magnetically stirred to ensure homogeneous distribution of the reagents; the stirring speed was set to 400 rpm throughout the experiments at room temperature (22 ± 2 °C).

The experiments began with the preparation of an aqueous AML solution; concentrations ranged from 20 mg/L to 250 mg/L, and all trials used 300 mL of solution. The pH of the solution was adjusted to 2.7 using a 1 M solution of sulfuric acid (H₂SO₄), as this condition is optimal for the Fenton reaction. FeCl₃ was then added as the Fenton reagent at different concentrations ranging from 14 mg/L to 75 mg/L, followed by the addition of H₂O₂ to initiate the reaction. The concentrations were chosen at levels that are both realistic and that would allow measurable degradation over a period of 15 min. The reaction was conducted under continuous stirring.

The anodic oxidation process was carried out using a cylindrical Aluminum bar, with a defined surface area of ≈$2.544{\mathrm{cm}}^2$. A direct current (DC) power supply equipped with voltage and amperage regulators was connected to the electrodes, and an electric potential of 5 V was applied between the anode and cathode. After each experiment, the surface of the electrodes was washed to remove any trace of acid on their surfaces [30]. The treatments were performed at a pH of around 2.7, considered optimal, and at 35 mg/L of FeCl₃, as shown to be optimal for the Electro-Fenton process. Sodium sulfate (Na₂SO₄) and sodium chloride (NaCl) powders were used as the electrolyte solution, and their effects were studied with vigorous stirring using a magnetic stirrer.

Both Fenton and electro-Fenton experiments were conducted in a batch reactor operated at room temperature. For the electro-Fenton process, anodic oxidation was carried out using a cylindrical aluminium bar as the anode, while the cathode was positioned opposite to ensure a uniform current distribution within the solution. The electrodes were connected to a DC power supply operating under a constant applied voltage. All experiments were performed under continuous magnetic stirring to maintain homogeneous mixing. Figure 1 presents a schematic representation of the experimental setup.

2.3. Analytical Methods

To determine the AML concentration of each sample throughout the Fenton and Electro-Fenton reaction time, samples were collected at different time intervals and immediately filtered to remove any suspended solids using a 0.45 µm PTFE syringe filter. A Cary 60 UV-Vis Spectrophotometer was used at a maximum wavelength of $\lambda =365$ nm. This technique was selected to enable rapid and reproducible monitoring of degradation kinetics during electro-Fenton treatment. Calibration curves were established over the investigated concentration range and showed good linearity. Potential interferences from iron species, electrolytes, and reaction by-products, were evaluated through blank and control experiments, which confirmed negligible absorbance contributions at the selected wavelength. While UV–Vis spectrophotometry is suitable for kinetic monitoring, the use of more selective techniques such as HPLC or LC–MS, is recommended for detailed identification of degradation intermediates and toxicity evaluation in future studies. The UV–Vis measurement at 365 nm is selective for amlodipine, as this absorption band originates from its dihydropyridine chromophore, which is rapidly destroyed during the initial ·OH radical attack. The oxidation intermediates formed afterward mainly consist of ring-opened aldehydes, carboxylic acids, and phenolic compounds that absorb below 300 nm and therefore do not interfere with measurements at 365 nm. This selectivity of UV–Vis monitoring for parent compound quantification has been widely reported in similar pharmaceutical degradation studies [31].

Additionally, the pH and conductivity of the solution were monitored throughout the experiment, using a FiveEasy pH meter F20. The following formula (6) is used to calculate the removal yield by degradation:

$$R\%={\displaystyle \frac{C_0-C_t}{C_0}}\times 100$$

(6)

where $C_0$ and $C_t$ are the concentrations (mg/L) of AML solutions before and after the reaction.

Energy consumption was estimated based on applied voltage (5 V), electrode surface area (2.544 cm²), and treatment duration. Specific energy consumption (SEC) was calculated as:

$$\mathrm{SEC}(\mathrm{kWh}/\mathrm{kg})={\displaystyle \frac{V\times I\times t}{m_{\mathrm{AML},\mathrm{removed}}}}$$

(7)

where V is the applied voltage (V), I is the current (A), t is the treatment time (h), and $m_{\mathrm{AML},\mathrm{removed}}$ is the mass of AML degraded (kg). Carbon footprint analysis encompassed three components: (i) electricity consumption (Algeria grid emission factor: 0.6 kg CO₂/kWh), (ii) chemical inputs (FeCl₃, electrolyte, H₂SO₄) using published life cycle inventory data, and (iii) electrode manufacturing and replacement amortized over estimated lifespan (50 cycles based on visual inspection and performance consistency across preliminary trials).

2.4. Aluminum Electrode Characteristics

The aluminium electrodes employed in the electro-Fenton experiments were made of commercially pure aluminum (≥99.5%). The material exhibits a polycrystalline microstructure with equiaxed grains and no detectable secondary phases. A thin native Al₂O₃ layer forms naturally on the aluminum surface, which becomes partially disrupted under the acidic conditions applied during the electro-Fenton process, facilitating efficient electrochemical reactions. Prior to use, the electrodes were mechanically polished, thoroughly rinsed with distilled water, cleaned with ethanol, and dried to ensure a reproducible surface state.

3. Results and Discussion

3.1. pH Effect

The initial pH is indeed an essential variable in advanced oxidation processes, particularly in Fenton-based systems, as it directly affects iron speciation, hydroxyl radical generation, and pollutant degradation kinetics [32]. In this work, the degradation of amlodipine (AML) via photolysis and Fenton oxidation was comparatively investigated over a wide pH range (3–10) for 150 min. Initial pH levels were adjusted using H₂SO₄ and NaOH [33,34].

Photolysis alone resulted in weak degradation, with efficiencies ranging from 0.23% to 38.93% and a maximum at pH 3. The above is consistent with previous literature reporting that photolysis usually cannot effectively degrade persistent pharmaceuticals, as their main structures are aromatic or heterocyclic and thus exhibit high resistance to direct photochemical cleavage [9]. As shown in Figure 2, under acidic conditions, a sharp increase in AML degradation was observed, whereas at neutral and basic pH levels, only a very slight photolysis occurred [35].

Conversely, the Fenton process was far more effective, achieving up to 97.73% degradation at pH 2.7 and 97.70% at pH 5.6. The high efficiency of the Fenton process at acidic pH indicates optimal conditions for the Fe²⁺/H₂O₂ interaction, ensuring continuous generation of hydroxyl radicals (·OH), the major oxidative species responsible for pollutant degradation [34]. The efficiency decreased with increasing pH, reaching 59.51% at pH 9, which could be explained by the precipitation of ferric ions as Fe(OH)₃, thereby decreasing the concentration of soluble Fe²⁺ available for catalysis [14,26]. The biphasic kinetics observed at pH 2.7 (Fenton, red curve) reflects two reaction phases: Phase 1 (0–5 min): rapid oxidation of parent compound by ·OH ($k_1\approx 0.28$ min⁻¹); Phase 2 (5–30 min): slower intermediate mineralization ($k_2\approx 0.08$ min⁻¹) due to Fe²⁺ depletion and radical scavenging by intermediates.

These results confirm that a sufficiently acidic environment, pH ≈ 2.5–3.0, is necessary to ensure high Fenton activity due to the most favorable iron redox cycling and hydroxyl radical availability [12]. This trend is clearly shown in Figure 2, with the low pH values contrasting the limited photolytic degradation with the pronounced oxidative performance of the Fenton process.

These results highlight the pH dependence of both processes, with the Fenton process being particularly effective under acidic conditions, where optimal hydroxyl radical generation occurs. The superior performance of the Fenton process underscores its potential for treating pharmaceutical-contaminated wastewater, while the limitations of photolysis suggest the need for more robust advanced oxidation processes [28,36].

Although the electro-Fenton process performs best under acidic conditions (pH ≈ 2.7), it should be noted that these conditions differ from those of most real wastewaters, which are generally near neutral. The need for pH adjustment and neutralization after treatment can increase operational complexity and costs at large scale. In this study, acidic conditions were deliberately chosen in order to explore the intrinsic kinetics and mechanistic behavior of amlodipine degradation under optimal electro-Fenton conditions. However, recent advancements in electro-Fenton technologies, including heterogeneous catalysts and iron-complexing strategies, offer promising ways to broaden the operational pH range and reduce chemical use. Future research will aim to adapt the process to more practical pH conditions to improve its industrial potential.

3.2. Effect of Initial Concentration of Amlodipine (AML)

Experiments were conducted to investigate the impact of initial AML concentration on its degradation efficiency in both the Fenton and Electro-Fenton processes. Figure 3 shows that pollutant concentration is one of the main operating parameters that affect the kinetics and degradation efficiency.

In this case, for the conventional Fenton process, under lower initial AML concentrations (20–30 mg L⁻¹), degradation rates were very rapid, reaching 76.6% within the first minute and peaking at 96.65% after 10–15 min. This is explained by the high availability of ·OH radicals relative to the pollutant load, which favors rapid oxidation reactions [1,22,37]. When higher initial AML concentrations (50–100 mg L⁻¹) were used, the removal rate decreased, likely due to the limited amount of ·OH radicals generated relative to the greater organic load, with competition for active sites [36,38,39].

Similar behavior was observed for the Electro-Fenton process. At 30 mg L⁻¹, AML degradation reached 39.1% after just 1 min and attained a maximum of 86.31% within 10 min, showing high oxidative activity at low pollutant concentrations. At a moderate level of 50 mg L⁻¹, the degradation efficiency increased gradually to 88.07% after 20 min. On the other hand, at higher pollutant concentrations ranging from 100 to 250 mg L⁻¹, the process exhibited slower initial kinetics; for instance, 85.06% efficiency was observed at 15 min for 100 mg L⁻¹. Thus, this confirms that higher pollutant loads require longer treatment times due to decreased ·OH availability and increased radical scavenging [40,41].

Since all other parameters were kept constant, including iron ion and H₂O₂ dosage, current intensity, and inter-electrode distance, the integral quantity of generated hydroxyl radicals remained fixed. Thus, with increased AML concentration, the probability of radical-pollutant interactions was reduced, which decreased overall removal efficiency [26]. Interestingly, at very low initial AML concentrations (20–30 mg L⁻¹), degradation became more challenging to quantify in this study due to instrumental limitations in detection and the low frequency of ·OH–pollutant collisions. Apart from that, several other scavenging species might be present in solution at low pollutant levels, including carbonate, chloride, and natural organic matter, which could further inhibit degradation [42]. This result highlights a general drawback of AOPs: low pollutant concentrations can significantly reduce the effectiveness of ·OH utilization. The development of hybrid AOP configurations or more sensitive analytical tools should be further investigated to enhance the degradation of trace pharmaceutical contaminants in water matrices [27,40,43].

3.3. Effect of the Concentration of FeCl₃

In the Fenton process, Fe²⁺ acts as a catalyst in the presence of hydrogen peroxide to generate hydroxyl radicals (·OH), highly reactive oxidants capable of degrading complex organic molecules. Since Fe²⁺ is frequently electro-regenerated from Fe³⁺ in electro-Fenton systems, its availability is crucial, as it directly influences treatment efficiency. Therefore, before scaling up, laboratory studies should optimize the concentration of ferrous or ferric ions to maximize electro-Fenton performance [16,38,44]. Conversely, a higher concentration of iron species increases the solution’s ionic strength, thereby improving current efficiency in an electro-Fenton system [16,36,45].

As shown in Figure 4, experiments were conducted with FeCl₃ concentrations of 0.5%, 1%, and 2%, and AML degradation was monitored for the first 30 min of the reaction. The initial 30 min of the Fenton process showed significant differences in degradation efficiency across FeCl₃ concentrations. At the lowest concentration (0.5%), the degradation efficiency increased steadily, reaching 41.45% at 5 min and 69.95% at 30 min. This gradual increase suggests controlled, sustained generation of hydroxyl radicals, consistent with optimal utilization of Fe²⁺ at lower concentrations [41]. With 1% FeCl₃, the removal rate improved more rapidly, reaching 53.9% at 10 min and 87.3% at 30 min, demonstrating the optimal catalytic activity [31]. The highest initial removal rate was observed with 2% FeCl₃, where the efficiency rises quickly to 82.1% within just 5 min. However, after this point, the rate plateaued slightly, dropping to 79.8% at 30 min, suggesting that excess iron may exert a scavenging effect, in which Fe²⁺ consumes hydroxyl radicals, reducing their availability for AML degradation [46]. The two-segment behavior (red curve) reflects initial rapid parent compound oxidation followed by slower intermediate mineralization, characteristic of advanced oxidation processes. These results align with prior studies that report optimal Fenton degradation performance at moderate iron concentrations, where excessive Fe²⁺ can inhibit the process by forming complexes or generating side reactions [16].

3.4. Effect of H₂O₂ Concentration on AML Removal

The hydrogen peroxide (H₂O₂) concentration is one of the key parameters in the Electro-Fenton reaction (electrochemical generation of H₂O₂ with Fe²⁺) as it directly influences hydroxyl radical (·OH) production while also influencing undesirable side-reactions (·OH + ·OH → H₂O₂; ·OH + H₂O₂ → HO₂· + H₂O) which negatively influence the process. Excess H₂O₂ may reduce removal efficiencies by scavenging radicals [41]. In this study, three H₂O₂ concentrations (50, 100, and 200 mg/L) were investigated during the first 30 min of treatment. The results revealed that the removal effect was not linear with respect to H₂O₂ concentration (Figure 5), as at 100 mg/L, the AML removal efficiency after 20 min was ∼91.3%, indicating controlled ·OH production without excessive H₂O₂ scavenging of radicals produced by the Electro-Fenton reaction. Comparatively, 200 mg/L produced suitable rates of removal (59.6% removal at 1 min), but overall, the rate plateaued much faster, which is consistent with observations in the literature that excess H₂O₂ promotes radical quenching, leading to diminished efficacy of the oxidation and possibly impacting removal efficiency of the intermediate species [47]. In addition, the observed decrease in removal efficiency after 20 min (8–12% decline) suggests depletion of Fe²⁺ and the accumulation of organic (intermediate) species produced by oxidation processes, consistent with previous observations in related Electro-Fenton systems [47].

The results obtained are supported by previous studies on Fenton and Electro-Fenton processes, which emphasize the need to optimize H₂O₂ concentration carefully. Mousavi et al. (2016) demonstrated that increasing H₂O₂ beyond the optimal level of approximately 11 mM (≈374 mg/L) reduced dye removal efficiency due to the scavenging of hydroxyl radicals [28]. Similarly, Omar et al. (2024) reported that H₂O₂ concentrations above 300 mg/L caused a plateau and, in some cases, a slight decline in the removal efficiency of synthetic composite industrial wastewater [29]. In the present study, we demonstrate that an H₂O₂ concentration of 100 mg/L provides an optimal balance between AML removal kinetics in surface-water matrices and minimizing losses due to excess H₂O₂ and radical scavenging. These observations highlight the importance of a process-specific adjustment that accounts for contaminant type, water matrix composition, and catalyst availability to determine the optimal H₂O₂ concentration [48].

Optimizing H₂O₂ dosage is an essential factor for the efficient operation of the Electro-Fenton process. Maintaining an appropriate Fe²⁺/H₂O₂ ratio ensures the maximum number of hydroxyl radicals are generated and helps minimize radical scavenging reactions, which may inhibit overall degradation efficiency. When the H₂O₂ is dosed in stages or controlled, hydroxyl radical generation can be sustained over a longer timeframe, helping to alleviate plateau values in pollutant removal that are otherwise found when dosing large amounts of H₂O₂ at once. The water matrix also plays an important aspect, as the presence of natural organic matter, inorganic ions, or other radical scavengers can alter the effective concentration of reactive species and modify the optimal dose of H₂O₂ for a particular system. From an economic standpoint, using 100 mg/L of H₂O₂ in the current AML treatment system provides an acceptable balance between H₂O₂ use and pollutant removal, avoiding excessive chemical requirements and residual H₂O₂ formation. Consideration of chemical dosing and water matrix composition is critical to maximize the efficiency and sustainability in the Electro-Fenton treatment process [9].

3.5. Effect of Electrolyte Type and Concentration

The form of the electrolyte and its concentration are critical parameters in the electro-Fenton process. In this study, both sodium chloride (NaCl) and sodium sulfate (Na₂SO₄) were tested, and the corresponding AML removal efficiencies at different concentrations are illustrated in Figure 6. The type of electrolyte and its concentration affect the AML’s degradation ability. For NaCl, the experimental concentrations were compared over 10 min. For NaCl at a moderate concentration of 50 mg/L, degradation was rapid, with 89.84% of the AML reduced in the first minute and reaching a maximum of 93.54% at 5 min. This indicates that there is an initial concentration of NaCl that enhances radical generation and pollutant degradation, due to higher conductivity and the formation of reactive chlorine species (Cl·, Cl₂·⁻) [4,24,49]. At a 75 mg/L NaCl concentration, degradation improved further to 96.00% after 10 min, indicating that too much NaCl does enhance degradation, but it may take marginally longer to achieve. However, at a low NaCl concentration of 25 mg/L and at an excessively high concentration of 100 mg/L, the efficiency was reduced: 87.71% after 10 min at the lower concentration, and the higher concentration degraded marginally less after 3 min. This decline at higher NaCl concentrations can be attributed to the scavenging of hydroxyl radicals by chloride ions and to the formation of less reactive chlorine species [26,41].

Conversely, Na₂SO₄ showed more stable degradation kinetics. Concentrations between 50 and 75 mg/L achieved a removal efficiency of around 92–93% within 15 min, mainly because of improved conductivity and in-situ generation of the reactive sulfate radicals SO₄·⁻ [19,50]. At higher concentrations of 100–200 mg/L, the process maintained efficiency at approximately 90–92% over longer reaction times without a significant scavenging effect, though slightly slower initial degradation rates were observed due to competition between sulfate and hydroxyl radicals at reactive sites [49].

These results confirm that sulfate-based electrolytes not only increase the conductivity of the reaction medium but also generate additional oxidizing species that can sustain pollutant degradation over time [50]. NaCl at an average concentration accelerates oxidation by forming reactive chlorine species (Cl·, Cl₂·⁻, HOCl).

In other words, NaCl and Na₂SO₄ enhanced the electro-Fenton degradation of AML, even as both presented different mechanisms. The addition of NaCl promotes rapid degradation by generating reactive chlorine species (Cl·, Cl₂·⁻), accelerating the reaction in the early stages, whereas Na₂SO₄ promotes degradation via the formation of sulfate radicals (SO₄·⁻), providing greater stability and sustained performance over longer operation times. However, they require longer times for complete degradation [8,51,52].

Consequently, all these findings emphasize the interrelationships among the type of electrolyte, pathways of radical generation, and pollutant characteristics in determining the efficiency and kinetics of advanced electrochemical oxidation processes (AOPs) [36,49].

3.6. Degradation Kinetics

According to

Table 1. Kinetic constants in different orders.

Kinetic Order	k	R2
Zero-order	4.76 min−1	0.89
Pseudo-first-order	0.15 min−1	0.99
Second-order	/	0.85

, the fitting of the results in the pseudo-first-order kinetic model, as observed from a high determination coefficient of R² = 0.99, evidences that this oxidation process is mainly governed by hydroxyl radicals ·OH in situ generated by Fenton reactions among ferrous ions and electrogenerated hydrogen peroxide [26]. The calculated apparent rate constant (k = 0.15 min⁻¹) and the corresponding half-life ($t_{1/2}$ = 4.62 min) are in close agreement with values reported for the EF degradation of structurally related pharmaceutical compounds such as atenolol, paracetamol, and diclofenac [3,11,37].

The fact that the pseudo-first-order model performed best when compared to the zero- and second-order models (R² < 0.90) suggests that the rate-determining step is not dependent on pollutant concentration but, rather, on the availability and steady-state generation of reactive oxidative species, predominantly hydroxyl radicals. A similar kinetic behavior has already been observed in EF degradation of a wide range of persistent organic pollutants, confirming that ·OH radical concentration, maintained by continuous H₂O₂ electro-generation and Fe²⁺ regeneration at the cathode, remains quasi-stable during operation [20,37]. This stability is an essential factor for maintaining efficient degradation kinetics, especially in complex pharmaceutical matrices, where radical scavenging may compete with oxidation pathways.

From a mechanistic point of view, pseudo-first-order kinetics are characteristic of systems in which pollutant oxidation is mainly controlled by the constant flux of hydroxyl radicals rather than by the direct concentration of the target molecule. Electrogeneration of H₂O₂ via oxygen reduction at the cathode (Equation (8)) and further activation of H₂O₂ by Fe²⁺ ions at the anode (Equation (9)) establishes a self-regenerating cycle of forming reactive species, thereby maintaining a quasi-steady-state ·OH radical concentration throughout the process. This explains the kinetic stability of the degradation curves and the reproducibility of the rate constants across successive runs [53].

$$\begin{array}{cc}\hfill {\mathrm{O}}_2+2{\mathrm{H}}^{+}+2e^{-}& \to {\mathrm{H}}_2{\mathrm{O}}_2\hfill \end{array}$$

(8)

$$\begin{array}{cc}\hfill {\mathrm{Fe}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2& \to {\mathrm{Fe}}^{3+}+·\mathrm{OH}+{\mathrm{OH}}^{-}\hfill \end{array}$$

(9)

The integration of aluminum electrodes into the EF setup yielded exceptionally high degradation efficiencies, with up to 97% AML removal under optimized operational conditions. This can be explained by the dual role of aluminum electrodes: they act as cathodes for H₂O₂ generation and as sacrificial anodes, which, by prompting electrocoagulation, couple oxidation and coagulation mechanisms [54]. The released Al³⁺ ions, during anodic dissolution, react with hydroxide ions, leading to the formation of amorphous Al(OH)₃ flocs. These further contribute to the adsorption and precipitation of partially oxidized intermediates. This hybrid Electro-Fenton–Electrocoagulation synergy enhances overall removal efficiency while reducing the toxicity of residual organic species [55].

Despite these economic and operational advantages, aluminum electrodes have certain drawbacks, including limited H₂O₂ yield due to parasitic hydrogen evolution and surface passivation from oxide layer formation. However, in recent years, these challenges have been considerably overcome by developments in electrode modification. MnO₂-, CNT-, and graphene-coated aluminum electrodes achieved 30–40% increases in H₂O₂ generation efficiency, with improved corrosion resistance, extending electrode lifespan [39]. Furthermore, a higher density of active sites, enhanced oxygen reduction kinetics, and cathodic performance stabilization during continuous operation were achieved by surface structuring and activation via plasma or anodization treatments [16,34].

Among critical environmental issues is the valorization of Al³⁺-based sludge generated during operation. Rather than being considered waste, sludge can be recycled as a coagulant in subsequent water treatment stages or in sludge dewatering units, thereby encouraging a circular, low-waste approach to water treatment [55]. This aligns with recent trends in the design of sustainable electrochemical treatments for resource recovery and environmental compatibility [21].

In short, kinetic analysis indicates that AML degradation in the Electro-Fenton system follows pseudo-first-order kinetics and is governed by hydroxyl radical oxidation. This aluminum-based EF configuration provides not only fast and efficient removal of contaminants but also an integrated, cost-effective, and eco-friendly solution. The mechanistic synergy in continuous radical generation, metal ion regeneration, and electrocoagulation makes this system highly promising for large-scale applications in treating pharmaceutical wastewater.

3.7. Electrode Stability and Reusability

To evaluate electrode durability and process stability, twelve consecutive treatment cycles were performed using the same aluminum electrode pair under optimized conditions (pH 2.7, 35 mg/L FeCl₃, 100 mg/L AML, 5 V, and 15 min per cycle). Between cycles, the electrodes were rinsed with deionized water to remove residual acid and air-dried at room temperature. No chemical cleaning or mechanical polishing was applied. The electrodes demonstrated excellent stability, achieving removal efficiencies of 97.2%, 97.0%, 97.0%, 97.1%, 96.8%, 97.0%, 96.8%, 96.5%, 96.7%, 97.2%, 97.0%, and 97.0% for cycles 1–12, respectively (mean: 96.9 ± 0.22%). The minimal variation (range of 0.7%) and absence of systematic performance decline confirm robust electrode stability. The sustained performance reflects continuous surface renewal through anodic aluminum dissolution and acid-facilitated oxide layer dissolution, which prevent passive film accumulation. The demonstrated stability with minimal maintenance confirms the practical feasibility of aluminum-based electro-Fenton for industrial pharmaceutical wastewater treatment, offering reliable performance with low operational complexity.

3.8. Environmental Sustainability and Energy Efficiency Assessment

The environmental sustainability of the aluminum-based electro-Fenton system was evaluated by quantifying energy consumption and carbon footprint. Under optimized conditions (5 V, 15 min), the system consumed 0.32 kWh/m³ to achieve 97% AML removal, corresponding to 6.5 kWh/kg AML degraded. This efficiency stems from synergistic cathodic H₂O₂ generation, continuous Fe²⁺ regeneration, and electrocoagulation. The rapid kinetics (k = 0.15 min⁻¹, $t_{1/2}$ = 4.62 min) minimize energy requirements. Compared to BDD-based systems (2–5 kWh/m³) [12,56] and carbon-felt cathode EF (1.5–3 kWh/m³) [34,45], the aluminum system demonstrates competitive performance despite unmodified electrodes, attributed to moderate applied potential (5 V vs. 10–30 V) and enhanced mass transfer.

The total carbon footprint was 0.53 kg CO₂-eq/m³, representing a 60–75% reduction compared to conventional Fenton (1.8–2.1 kg CO₂-eq/m³) and ozonation (3.5–5.0 kg CO₂-eq/m³) [32,43]. This encompasses electricity (0.19 kg CO₂-eq/m³, 36%), chemicals (0.28 kg CO₂-eq/m³, 53%), and electrode manufacturing (0.06 kg CO₂-eq/m³, 11%). Environmental advantages stem from: (i) eliminating external H₂O₂ transport/storage (∼0.8 kg CO₂-eq/m³ avoided) [20], (ii) lower energy intensity than high-voltage systems, and (iii) using abundant aluminum (12 kg CO₂/kg) versus platinum (15,000 kg CO₂/kg) [21,55].

Economic analysis for 100 m³/day facilities reveals operational costs of $0.71–1.05/m³, compared to BDD-based EF ($3.50–5.00/m³), ozonation ($3.00–4.50/m³), and activated carbon ($1.80–2.50/m³) [2,12,16]. Aluminum electrodes cost 100× less than platinum ($5–8/kg vs. $50,000–80,000/kg), enabling capital investment that is 30–50% lower than for BDD systems, with 2–3 year payback periods [16,21]. Additionally, Al(OH)₃ sludge (45–60 mg/L) can replace 40–60% of commercial alum in downstream coagulation, offsetting $0.16–0.24/m³ and advancing circular economy principles [55]. This contrasts with Fe(OH)₃ sludge, requiring disposal at $0.15–0.45/m³ [57].

3.9. Degradation Pathway and Toxicity Considerations

The electro-Fenton degradation of amlodipine is primarily driven by hydroxyl radicals, which induce successive oxidation steps including hydroxylation, cleavage of aromatic and heterocyclic structures, as well as the fragmentation of the parent molecule [34,41]. These reactions lead to the formation of lower-molecular-weight intermediates, which are further oxidized under sustained electro-Fenton conditions. The high degradation efficiency and rapid kinetics observed in this study indicate that the process favors progressive oxidation rather than the accumulation of persistent intermediate compounds [41]. According to previous studies on pharmaceutical electro-Fenton degradation, such transformation pathways are generally associated with a reduction in overall toxicity compared to the parent compounds [36]. Nevertheless, further investigations combining advanced analytical identification and ecotoxicological assays are recommended in order to fully assess the environmental safety of the treated water [36].

4. Conclusions

This study demonstrates that an aluminum-based electro-Fenton system rapidly and efficiently degrades amlodipine—a persistent pharmaceutical pollutant of growing environmental concern. Under optimized conditions (pH 2.7, 35 mg/L FeCl₃, 1.3 mM NaCl, 5 V), the system achieved 97% AML removal within 15 min, following pseudo-first-order kinetics (k = 0.15 min⁻¹, $t_{1/2}$ = 4.62 min). The degradation mechanism is governed by hydroxyl radical (·OH) oxidation, with synergistic contributions from electrocoagulation via Al³⁺ release and continuous Fe²⁺ regeneration at the cathode.

Comprehensive sustainability assessment revealed exceptional environmental and economic performance: energy consumption of 0.32 kWh/m³, carbon footprint of 0.53 kg CO₂-eq/m³ (60–75% lower than conventional AOPs), and operational costs of $0.71–1.05/m³ (30–50% lower than BDD-based systems). These advantages stem from the use of abundant, low-cost aluminum electrodes ($5–8/kg) rather than precious metals ($50,000–80,000/kg), moderate applied potential (5 V vs. 10–30 V in BDD systems), and the potential for Al(OH)₃ sludge valorization as a recoverable coagulant.

This work advances the field of sustainable pharmaceutical wastewater treatment by demonstrating that high-performance electrochemical remediation is achievable with Earth-abundant materials, thereby challenging the prevailing reliance on expensive, energy-intensive precious metal electrodes. The aluminum-based EF configuration, integrating cathodic H₂O₂ generation, anodic electrocoagulation, and Fe²⁺ regeneration in a single reactor, reduces system complexity, lowers capital and operational costs, and achieves a competitive carbon footprint with respect to the most environmentally responsible treatment technologies.

The demonstrated scalability, economic viability, and compatibility with existing wastewater infrastructure position this technology as particularly suitable for deployment in resource-constrained settings, including small- to medium-sized pharmaceutical manufacturers, hospital campuses, and decentralized treatment facilities, where cost considerations often preclude the adoption of advanced AOPs. By demonstrating that environmental protection and economic accessibility can coexist in advanced wastewater treatment, this work lays the foundation for next-generation sustainable strategies to control pharmaceutical pollution.