Removal of pharmaceuticals and personal care products from wastewater via anodic oxidation and electro-Fenton processes: current status and needs regarding their application

Pharmaceuticals and personal care products (PPCPs) are constantly diversifying and increasing in production (Khan et al. 2022). Due to their potential human and ecological effects, they are considered emerging contaminants (ECs). Pharmaceuticals mainly consist of antibiotics, analgesics, hormones, anti-inflammatory drugs, beta-blockers, among others, while personal care products include chemicals used for cosmetics, surfactants, food additives, hygiene products, and fragrances (Pai & Wang 2022). Moreover, metabolites and transformation products of PPCPs generated in the environment or during their removal are also considered ECs (Zhang et al. 2022). Many of these contaminants are also considered recalcitrant to conventional water treatment (Patel et al. 2019).

The presence of PPCPs in aquatic and land ecosystems has been found to be linked to their existence in the influents and effluents of wastewater treatment plants (WWTPs). In wastewater streams, the concentration of PPCPs reaches up to 10,000 ng/L (for analgesics in Africa) and its concentration decreases between 10 and 90% (Adeleye et al. 2022). Several factors influence the removal of PPCPs during wastewater treatment, such as the pollutant's chemical structure, concentration, and solubility, as well as the technology utilized for treatment. Activated sludge and anaerobic digestion are the prevailing technologies employed in wastewater treatment. Nevertheless, these methods typically exhibit limited efficiency in degrading PPCPs (Hidrovo et al. 2022).

In some cases, conventional treatments such as activated sludge have shown partial success in removing pharmaceuticals like diclofenac, ibuprofen, and triclosan, particularly in acclimated cultures (Kim et al. 2017). However, this process is time-consuming, resulting in relatively low degradation rates (Patel et al. 2019). Consequently, using anaerobic processes for the biodegradation of other PPCPs may require larger treatment units, leading to increased costs. Overall, both aerobic and anaerobic conditions present several limitations for the complete removal of PPCPs through biodegradation. On the other hand, emerging treatments such as electrochemical advanced oxidation processes (EAOPs) have shown great potential for PPCP degradation under certain operational conditions (Moreira et al. 2017; Patel et al. 2019; Pai & Wang 2022).

EAOPs are a promising up-and-coming type of water treatment that has been extensively studied in recent decades (Klavarioti et al. 2009; Barrera-Díaz et al. 2014; Sirés et al. 2014; Moreira et al. 2017; Lozano et al. 2022). Many efforts have been put toward studying the degradation capabilities of EAOPs in synthetic and, to a lesser extent, real water matrices. There are some features that make EAOPs an attractive alternative for organic degradation, including PPCPs: the experimental setup is relatively simple, especially for non-radiation systems such as anodic oxidation (AO) and electro-Fenton (EF) (Feng et al. 2013). In these processes, operational costs are mostly comprised of electric energy consumption (Muddemann et al. 2021). Moreover, high degradation and mineralization efficiencies can be achieved by a combination of direct oxidation (AO) and oxidants (mainly ^•OH)-mediated oxidation (Hu et al. 2021), abating of persistent organic pollutants, and a lack of additional reactants. Despite the many aspects that make EAOPs a very promising technology (Moradi et al. 2020), there is a clear lack of real-life application, and commercially available water treatment units in the field are scarcely reported. In this review, we highlight some of the most recent studies on the removal of PPCPs via AO and EF-based processes, their operating conditions, trade-offs, as well as some of the techno-economic aspects facing their realistic applications.

The mechanism of action in EAOPs involves the simultaneous application of electrochemical oxidation and advanced oxidation techniques, leading to the generation of powerful oxidants that facilitate the degradation of organic pollutants in water and wastewater. During EAOPs, electrochemical reactions occur at the surface of specially designed electrodes, resulting in the formation of highly reactive species, such as hydroxyl radicals (^•OH). These radicals possess strong oxidizing potential and exhibit a non-selective attack on organic compounds, breaking down complex molecules into simpler, less harmful byproducts. Additionally, EAOPs can promote the regeneration of oxidants through the in situ electrogeneration of H₂O₂ and other active oxygen species. The synergistic effects of the generated oxidants, coupled with the enhanced mass transfer at the electrode–electrolyte interface, ensure efficient pollutant degradation even at low concentrations. Moreover, the use of various electrode materials and configurations in EAOPs enables fine-tuning of the process to suit specific applications, making it a versatile and promising technology for sustainable water treatment. In the following subsections, we address the main characteristics of some of the most representative types of EAOPs, namely AO, EF, and photo-electrocatalysis.

AO is one of the most direct methods to remove several types of PPCPs, mainly antibiotics such as amoxicillin (Sopaj et al. 2015), cefalor (Kurt 2020), cefazolin (Sordello et al. 2021), ciprofloxacin (Lima et al. 2020), sulfonamides (Fabiańska et al. 2014), sulfamethoxazole (Loos et al. 2018), tetracycline (Oturan et al. 2013), and trimethoprim (González et al. 2011), among others. AO can be described as an electrochemical oxidation process that occurs on the anode's surface, or in a mediated oxidative process, or both (Klavarioti et al. 2009). The oxidative process of PPCPs in water can be heterogeneous and homogeneous. The heterogeneous process occurs when the PPCPs are directly oxidized in the electrode surface involving the following steps: transport from the solution to the surface of the electrode, adsorption of the molecule, and electron transfer to the pollutant combined with direct electrochemical reaction, desorption of the products, and transport to the water matrix. Homogeneous oxidation of the PPCPs in water is mediated by the oxidants produced on the anode surface from components present in the matrix, or other ions added to the media to promote oxidation (Panizza & Cerisola 2009; Feng et al. 2013). However, byproducts formation during degradation of PPCPs when they are not fully mineralized may have similar or more adverse effects to the aquatic organisms than the original compound.

Hydroxyl radicals (^•OH), generated either from water or hydroxide ions oxidation, are the strongest oxidants and are readily produced in AO (Sirés et al. 2007). However, weaker oxidants such as H₂O₂ from ^•OH dimerization and O₃ are also produced from water (Kapałka et al. 2009), complementing both anodic processes in a highly efficient way: the direct oxidation of pollutants on the anode surface, and the formation of ^•OH and subsequent pollutant oxidation. Since hydroxyl radicals have a short lifespan (Yang et al. 2017), the oxidation of PPCPs mainly occurs in the anode surface and its vicinity, representative of the heterogeneous nature of the AO treatment. Therefore, anode selection is crucial to successfully perform the degradation processes until the mineralization of PPCPs in wastewater treatment.

Boron-doped diamond (BDD) electrodes can be found in the literature as the most successfully applied anode material for PPCPs degradation (García-Montoya et al. 2015; Moreira et al. 2017; Loos et al. 2018; Lozano et al. 2022). This trend can also be seen in the compilation of the last 5 years studies presented in Table 1; BDD electrodes are chosen mainly because they possess the largest known oxygen overpotential range about 2.2–2.6 V vs. standard hydrogen electrode (SHE) which allows them to be operated in the potential range of water decomposition to form hydroxyl radicals, some of the strongest oxidizing agents while minimizing oxygen evolution (Enache et al. 2009; Moreira et al. 2017). High contaminant (PPCPs) concentrations, ranging from 0.5 to 100 mg L⁻¹ with 50 mg L⁻¹ can be treated, which is greater than the concentration detected in most water bodies and wastewaters, which ranges from ng L⁻¹ to μg L⁻¹ (Ebele et al. 2017; Hena et al. 2021).

The comparisons of many electrodes in the market tend to favor BDD electrodes as the best choice for AO, which is four times better than dimensional stable anodes (DSA), such as steel/IrO₂–Sb₂O₃ and Ti/IrO₂–Sb₂O₃, for degradation of amoxicillin (Sopaj et al. 2015), and is more efficient in removing tetracycline over time than Pt and Ti/RuO₂ electrodes (Oturan et al. 2013). BDD has also been reported as the best option for complete mineralization of caffeine when compared to PbO₂ and Pt electrodes with 40 and 15% of mineralization, respectively (Chen et al. 2015). Numerous studies indicate that BDD electrodes are the best all-around choice for organic pollutant degradation (Kapałka et al. 2009; Garcia-Segura et al. 2012; Ambuludi et al. 2013; Chen et al. 2014; Sopaj et al. 2015; Kouskouki et al. 2019); however, full mineralization is highly recommended to avoid undesirable byproducts formation. The most commonly reported oxidation byproducts are carboxylic acids (Ganiyu et al. 2018; Cai et al. 2020), although oxidation byproducts are not always reported due to the complexity of detection and quantification, and they can vary substantially based on experimental conditions such as current density (50–200 mA cm⁻²), water media, pH (2.5–7), time (20–180 min), among other factors. Most studies have used Na₂SO₄ (5–100 mM) as electrolytic media based on its inert and neutral nature to avoid pH changes and secondary chemical reactions (Table 1). In addition, sulfate and some anions (such as phosphate and carbonate), promote the formation of stable oxidants such as and for BDD and some PbO₂-coated electrodes (Sirés et al. 2014). Further research using real water and wastewater matrices is needed to scale up AO processes and to better understand the mass transport of the pollutants from the water matrix to the anode surface and its vicinity under more realistic conditions.

The type of pollutant also influences the operating conditions for PPCPs degradation via AO. For example, the removal of diclofenac was higher than iopromide under the same conditions (Loos et al. 2018), suggesting that increasing the current density or time of exposition removal could enhance. This is because iopromide has more peptide (amide) bonds than diclofenac, requiring more energy to be degraded. Another polar covalent bond that can be challenging to break down through AO is the C–F which corresponds to per- and polyfluorinated substances, which are present in cosmetics and may potentially reach wastewater. The mineralization of these and other highly stable chemical substances is needed to promote the application of the AO process.

Electrochemical oxidation processes based on the Fenton reaction, also known as EF processes, are some of the most sought-after EAOPs to this day, given their high mineralization potential of organic compounds (Panizza et al. 2014). The basis of EF is the Fenton's reaction in which Fe²⁺ is oxidized to Fe³⁺ by electrogenerated H₂O₂, forming hydroxyl radicals (^•OH) and a hydroxide ion (OH⁻).

EF processes can be assisted with solid catalyst which is known as a heterogeneous EF (H-EF) process, and by UV radiation such as photoelectron-Fenton (PEF) and solar photoelectron-Fenton (SPEF). In the case of heterogeneous processes, the aim is to promote ^•OH formation from H₂O₂ and Fe²⁺, while in the UV-assisted processes, Fe³⁺ complexes formed are photoexcited to produce both Fe²⁺ and ^•OH increasing efficacy overall and avoiding unnecessary reactions (Lozano et al. 2022).

To achieve the best performance for EF processes, a careful selection of both electrodes (anode and cathode) (da Silva et al. 2021; Olvera-Vargas et al. 2021a) and the experimental conditions between them must be conducted (He & Zhou 2017). The most common cathodes are carbon-based, while the materials for anodes that can degrade PPCPs have been BDD, Pt-, Ti-, RuO₂- and IrO₂-based, either as mesh or sheet (Table 2). The production of ^•OH is the main difference for all anodes. In general, BDD, PbO₂ and IrO₂-based anodes result in a higher removal of PPCPs (Droguett et al. 2020). Other factors influencing the removal of PPCPs are the experimental medium, usually Na₂SO₄, ranging from 5 to 100 mM, pH between 3 and 4, current density ranges between 3.8 and 150 mA·cm⁻², as well as a certain amount of catalyst in the case of heterogeneous process and irradiance (1.1–55 W·m⁻²) for PEF and SPEF processes. Full removal of PPCPs in a short time (from 20 to 360 min) can be achieved at concentration ranging from 0.1 to 0.6 mM with some trade-offs such as pH, catalyst, and current density.

More research is needed that focuses on removal mechanisms, optimization of catalysts and influence of interferences in real wastewater such as radical scavengers and ionic strength, among other factors. The main expected outcomes are the optimization of the operating conditions such as pH, current density, Fe²⁺ and H₂O₂ concentration, O₂ flow rate, and electrode gap distance. However, little is known about the application of new working electrodes made of unconventional carbon sources such as, organic fraction of urban waste or agricultural waste, which will decrease the net environmental impact of this EF process. Moreover, moving from assisted to natural radiation supply in the case of PEF and SPEF, respectively, is a highly relevant factor that influences the sustainability of EF-based technologies applied for PPCP removal.

There are many aspects to consider when deciding the type of technology to use for water treatment, such as the type of contaminants to treat and regulatory requirements. However, for any new technology to either compete or find a niche application, one of the most important aspects to consider is economic viability. When considering the economic viability of EAOPs, one can study aspects such as: cost of electrodes, energy consumption, cotreatments, and synergistic effects.

BDD is one of the most sought-after electrode materials for many types of EAOP. Recently, Lozano et al. (2022) reviewed the application of EAOPs for the removal of PPCPs and found that BDD was the most commonly used material in the AO studies reviewed, given its exceptional capacity for ^•OH production and chemical stability (Oturan 2021). More recently, BDD has also been increasingly studied in EF–AO combination processes for PPCP degradation due to the high synergy found between techniques (Cai et al. 2020; Zhu et al. 2020; Olvera-Vargas et al. 2021b; Oturan 2021; Mbaye et al. 2022). Oturan (2021) recently reviewed the impact that BDD anodes have on the performance of EF processes and found a clear and strong enhancement in both degradation and removal efficiency.

Although BDD shows a very marked advantage in degradation and removal efficiency over other types of anode materials (He et al. 2019), using BDD can significantly increase treatment cost. Recently, Stirling et al. (2020) used a techno-economic analysis to identify what they describe as key innovations to increase electrochemical oxidation competitiveness in point-of-use treatment systems. Their results identified electrode material as one of the main elements that are driving capital costs and concluded that a tenfold decrease in BDD cost and exploring alternative emergent materials such as Ti₄O₇ can significantly improve affordability of electrochemical devices in the market. Salazar-Banda et al. (2021) reviewed the most recent advances in the development of novel materials for both anodes and cathodes and found significant advancements in novel electrode preparation involve new heating methods, different solvents, and modifications in their compositions. Even when these modifications to electrode preparation clearly show an improvement in efficiency, stability and electrocatalytically activity, Salazar et al. concluded that additional efforts in large-scale trials are necessary to accurately test the applicability of these materials (Salazar-Banda et al. 2021).

One of the most important parameters to consider when determining the viability of an electrode material is its estimated lifetime, given that it directly impacts initial investment and operational costs when considered as a consumable. In their techno-economic analysis, Stirling et al. (2020) considered electrode lifetime as a key assumption in their calculations for an EAOP point-of-use device. Although electrode lifetime is clearly a very important factor when determining an EAOP economic viability, not much information is available regarding this subject. In this regard, the review published by Moradi et al. (2020) focuses on electrode lifetime as a key parameter for further EAOP development. Moradi et al. describe several aspects that affect electrode lifetime such as: applied current density, soluble compounds, method of preparation, electrode shape, pH, temperature, and in the case of composites bonded to a substrate, they identified oxygen bubbles and the creation of high-pressure zones at the electrode surface, since they are the main sources of corrosion during EAOP operation. Given the time consuming and highly difficult task of obtaining real service lifetime values for novel electrodes, Moradi et al. point to accelerated lifetime tests under strong acidic conditions (0.5 M H₂SO₄) with a high current density (200 mA cm⁻²) as way to estimate the approximate service lifetime of electrodes (Lim et al. 2018; Yao et al. 2019; Zhang et al. 2019a; Xia et al. 2020). Data analysis demonstrated that BDD, TiN, and IrO₂ doping significantly increased the lifetime of PbO₂ and DSA electrodes, reaching between 3.9 and 10.4 years of service (Moradi et al. 2020).

Although electrode costs tend to be the bulk of the economic burden in EAOPs (especially for BDD-based processes), operational costs can also diminish commercial interest in EAOP application. Current efficiency is the main parameter to consider for assessing operational costs, as energy consumption can more than quadruple for processes that operate at high current densities where the process is controlled by mass transfer (Radjenovic & Sedlak 2015). Many efforts have been made to address this problem, from new enhancements in electrode preparation (Hu et al. 2021; Salazar-Banda et al. 2021) to new reactor designs like the one recently reported by Muddemann et al. (2021) in which a process involving a novel BDD anode deposited on a tantalum substrate and a gas diffusion electrode (GDE) as cathode are used to treat an artificial wastewater sample containing phenol and showed that the novel BDD–GDE system can achieve complete mineralization with a reduction in energy consumption of 25, 86, and 92% when compared with BDD-stainless steel, ozonation, and peroxonation systems, respectively. The increase in energy efficiency can be attributed to an improvement in the mass transfer conditions given the 3D structure of GDE electrodes and the absence of the hydrogen evolution reaction which doesn't contribute to contaminant degradation. Muddemann et al. also estimated a lifetime of approximately 18 years for the novel BDD electrode based on surface corrosion analysis, and through an operational expenses (OPEX) analysis of a case study comparing with classic niobium-based BDD electrodes, they found that replacement costs can be diminished by a factor of 5, effectively reducing operational costs.

The coupling of EAOPs with synergistic processes, such as photo and ultrasonic irradiation, often leads to an increase in degradation and/or removal efficiency, and it is a very promising and researched subject that merits its own classification in EAOP technologies. EF processes can be enhanced by the irradiation of UV light (PEF) by increasing Fe²⁺ generation due to Fe(OH)₂⁺ photoreduction and photodecarboxylation of Fe(III)-carboxylate species (Brillas 2020). However, the high cost of artificial UV lamps, both initial and operational, represents an important obstacle for applying PEF processes at a larger scale. Synergistic analysis considering both degradation efficiency and energy consumption can help determine to what extent photo-irradiation is worth the additional electric consumption as the additional costs can be several times greater than non-irradiated EAOPs (Khataee et al. 2012). Photo-irradiation operational costs can be reduced using solar driven EAOPs during some seasons, and in some locations, the great potential of free, renewable, and environmentally friendly solar radiation makes them very attractive for degradation of any organic pollutant, including PPCPs (Brillas 2020; Lozano et al. 2022). One of the main drawbacks of solar driven EAOPs is the relative lower degradation efficiencies obtained when compared to UV irradiation since most semiconductors used, such as TiO₂, work better under UV irradiation (approximately 420 nm). In recent years, several efforts have been made to increase the effectiveness of solar radiation in EAOPs. In this regard, various novel semiconductor electrodes considering heteroatom doping and composites with enhanced absorption ability to visible light have increased the viability of solar driven EAOPs (Brillas 2020; Ye et al. 2021; Lozano et al. 2022). Recently, Karthikeyan et al. (2020) did in-depth review of the recent advances in some of the most popular photocatalysts such as TiO₂, ZnO, WO₃, CuO, and SnO₂, reporting that minimizing the band gap energy and electron–hole recombination rates are key elements in further development of photocatalysts. They also noted that large-scale application requires the exploration of novel preparation methods that are facile, efficient, and cost-effective.

Ultrasound (US) radiation can also have a synergistic effect with EAOPs and significantly improve degradation efficiency (Lozano et al. 2022). However, US technology is quite costly, and one must keep in mind that the benefits may not be worth the additional investment, both in the substantial initial cost of acquiring US equipment and the additional operational costs related to energy consumption. In this regard, careful optimization of operating parameters must be conducted to determine the conditions in which US radiation is justified, such as the recently published study by Kacem et al. (2022) in which they determined that US radiation could be used for the degradation of a model compound (Mordant orange-1) when in the presence of sulfate ≥20 mM for the entire process or just for the initial part when sulfate is not present. Nevertheless, they found that synergistic effects tend to be higher for low current values.

Some of the most promising applications of EAOPs lies within their highly versatile nature, which is mainly explored when they are used as a cotreatment to achieve a certain objective, such as meeting a discharge criteria. There are many examples of EAOPs employed as an additional treatment. Recently, Feng et al. (2021) published a study in which they treated landfill leachate using a combination of EAOPs (BDD anodes), biological treatment (anoxic–aerobic), and ultrafiltration. Feng et al. found that the highest current efficiency occurred at low current values (1 mA/cm²). Most interestingly, they analyzed the impact of different operational conditions (mainly current density) on the overall operational cost and found that combining EAOPs and biological treatment had the lowest operational cost (0.76 USD/m³) of all the technologies they analyzed. Additionally, they found that the highest cost savings (up to 54%) were achieved for the lowest current density value studied, which corresponds to the highest current efficiency, in accordance with some of the aspects discussed in the previous subsection.

EAOPs are undoubtedly a very promising set of technologies that could drastically change the way water treatment is done in the next decade. There is no clear unique research path to make EAOPs more competitive, and current research seems to be correctly focusing on all the factors that could increase EAOP's market share, including their application to specific PPCPs such as antibiotics, anti-inflammatory drugs, and others. Given the potential problems generated by transformation products during EAOPs, complete mineralization is an important matter for EAOP further development; however, more efforts such as the one recently proposed by Stirling et al. (2020) to identify key innovations through techno-economic analysis or case studies like the one proposed in the recent publication of Muddemann et al. (2021) using OPEX analysis to study the economic viability of novel reactor configurations are needed to help complement the many studies already available on EAOP novel materials and their application on specific pollutants (He et al. 2019; Karthikeyan et al. 2020; Hu et al. 2021; Salazar-Banda et al. 2021; Lozano et al. 2022). Finally, accurate data on key elements of EAOPs such as electrode lifetime (Moradi et al. 2020) are vital for further economic analysis.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.