Porous calcium copper titanate electrodes for paracetamol degradation by electro-oxidation via CuO-induced peroxymonosulfate activation

Abstract

Some drugs cannot be efficiently eliminated using routine wastewater treatments and therefore are considered persistent organic pollutants (POPs). POPs can be removed using advanced oxidation processes. Among these processes, the combination of electrocatalysis and a sulfate-based advanced oxidation process via peroxymonosulfate (PMS) activation is an attractive approach due to its high efficiency, low energy consumption and non-selective attack. It is well known that the type of anode strongly affects the electrocatalysis performance for water treatment. Here, we evaluated perovskites as electrode material due to their unique structural properties and high chemical stability. We fabricated porous anodes of calcium copper titanate (CaCu₃Ti₄O₁₂; CCTO) with different percentages (20%, 30% and 40%) of polymethyl methacrylate (PMMA) by ball-milling. The samples that included PMMA displayed 50% porosity and pores were homogenously distributed. Morphological measurements show the presence of grain structures and grain boundaries containing CCTO and CuO phases, respectively. CCTO with 30 wt% PMMA (CCTO-30) exhibited the highest CuO phase amount, defect percentage and oxidation–reduction peak, and the smallest resistance. We used the obtained CCTO nanocomposites as anodes in a beaker (210 mL) with PMS (0.5 mM) to treat 10 ppm paracetamol in 50 mM sodium sulfate. After 90 minutes, paracetamol was completely decomposed using CCTO-30 due to PMS activation by a copper catalytic cycle (Cu²⁺/Cu¹⁺ and Cu²⁺/Cu³⁺) to generate ˙SO₄⁻ radicals and Cu³⁺ non-radicals that are selective for its removal.

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Environmental significance

Water pollution, particularly from pharmaceuticals, poses significant risks to human health and ecosystems. Eliminating these pollutants at low concentrations is highly challenging. To address this issue, it is vital to develop purification systems that can effectively remove trace compounds. Advanced oxidation processes, such as electrocatalysis combined with peroxymonosulfate activation, have shown promise in this regard. These processes offer efficiency, cost-effectiveness, and environmental friendliness. However, the search for highly active catalysts for anodic oxidation remains ongoing. Calcium copper titanate, an electroceramic composite known as Ti-based perovskite, is a potential catalyst due to its unique structure containing both copper oxide and titanium dioxide phases. This composite could play a vital role in addressing water pollution challenges.

1. Introduction

Persistent organic pollutants (POPs) in water are becoming a major threat worldwide. Organic micropollutants are found in water at very low concentrations (μg L⁻¹–ng L⁻¹) but can have harmful effects on aquatic life and human health. POPs include many different organic compounds (e.g. drugs, pesticides/herbicides, and chemicals, such as flame retardants and plasticizers) that are used in everyday life. In particular, anti-inflammatories, lipid regulators, antibiotics, and analgesics are drugs used for the treatment, diagnosis and prevention of diseases and also to modify organic processes.^1–3 These substances are released into the environment mainly through the wastewater of pharmaceutical production facilities, hospitals, and human activities. Due to their stability and persistence in water, these pollutants adversely affect human health and ecosystems.^4,5 For instance, paracetamol, also known as acetaminophen, is a pain-relieving compound commonly consumed worldwide.² In aquatic ecosystems, paracetamol is harmful even at low concentrations.^6–9 Therefore, to prevent their potential toxicity, water recycling techniques must ensure adequate water decontamination by removing all drugs and their metabolites. However, conventional wastewater treatment plants only partially remove pharmaceutical contaminants and pose additional problems, including their need of chemical reagents and the formation of by-products or of sludge that is difficult to treat. As a result, many pollutants end up in the environment.^10–12 Among the alternative and more environmentally friendly methods, advanced oxidation processes (AOPs) are considered one of the most promising ways to effectively remove several contaminants by oxidizing organic pollutants into harmless compounds.^13–17 These methods allow the in situ generation of hydroxyl radicals that act as non-selective oxidizing agents to destroy non-biodegradable organic molecules in wastewater.^4,6,18–20 These AOPs include catalytic ozonation procedures, sulfate-based AOPs, and treatment with ultrasound, electro-catalytic oxidation, and photocatalysis.^12,21–24 Electrocatalytic oxidation has been extensively studied for wastewater treatment due to its simplicity, capacity to mineralize pollutants without producing secondary pollution, and high oxidation rates by losing electrons at the anode or by oxidizing reactive species, leading to the organic pollutant transformation into CO₂ and H₂O.^25–28 Direct anodic oxidation occurs at the electrode surface by electron transfer. Conversely, in indirect oxidation, first reactive oxygen species are electrochemically formed, and then, they efficiently oxidize organic molecules to produce small biodegradable organic molecules.^29,30 Electrochemical oxidation is influenced by many different variables, such as the applied electrolyte, pH, delivered power, and contaminant type(s) and concentration(s). The choice of anode also strongly affects the electrocatalytic degradation effectiveness.^31–34

For electro-oxidation, different electrode types can be used (e.g. boron-doped diamond, graphite, tin and ruthenium oxide) with different electron transfer ability and number of ˙OH radicals formed.³⁵ However, simple single-metal oxides often display low conductivity and limited application in anodic oxidation.³⁶ Perovskite oxides (ABO₃) are multi-metal oxides that have been thoroughly investigated as a suitable class of electrocatalytic materials due to their unique structure and composition. Perovskite oxides are particularly popular in electrocatalysis due to their (1) low-cost and simple synthesis techniques with excellent stoichiometric control, repeatability, and homogeneity and (2) high capacity to accommodate a wide variety of substituting and doping elements to regulate their properties.³⁷ For instance, Ma et al. synthesized a spinel Cu_xCo_1−xMn₂O₄ anode for the electrocatalytic degradation of tetracycline hydrochloride. After 120 minutes, the removal efficiency reached 91.3% with good stability after five cycles of anode reuse. These results can be attributed to the multi-metal oxides present in Cu_xCo_1−xMn₂O₄ perovskite that offer high electrocatalytic activity to enhance electron transfer. This leads to the generation of radicals that react with and remove organic pollutants.³⁷

CaCu₃Ti₄O₁₂ (CCTO) is a cubic (AA′BO₃) double-perovskite with Ca²⁺ and Cu²⁺ organized on the A and A′ sites, and with Ti⁴⁺ arranged on the B site. This material presents extremely high dielectric permittivity (10⁴–10⁵) at a wide range of frequencies (10²–10⁶ Hz) and temperatures (100–600 K). The CCTO structure is obtained by Ti⁴⁺ and open-shell Cu²⁺ incorporation into perovskite without doping. Jahn–Teller distortion in Cu²⁺ is caused by a distortion of the TiO₆ octahedrons in the structure that creates a square planar structure. Moreover, the strongly covalent interaction between oxygen atoms and A′-Cu and B-Ti ions enhances the charge transfer between them, which is very important for electrocatalytic applications.^38–42

In addition to altering the electrode material to improve POP electrochemical decomposition, changes in the active radicals also have been investigated, particularly sulfate radical-based AOPs that rely on the in situ generation of ˙SO₄ radicals through peroxymonosulfate (PMS) activation. This can be obtained using different methods, such as UV irradiation, catalysis by metal ions, electrochemistry, metal oxides, and carbon material.^23,43–45 Transition metals and their oxides are commonly used as activating species in sulfate radical-based AOPs. For instance, copper as an A′-site metal in CCTO perovskite is an effective catalyst for PMS activation for drug elimination. Indeed, Cu⁺–Cu²⁺ and Cu²⁺–Cu³⁺ redox pairs promote ˙SO₄⁻ generation via PMS activation.^46–48 Hence, compared with other transition metal oxides, copper oxide (CuO) is one of the most effective catalysts for decomposing PMS into ˙SO₄⁻, with low cost and limited toxicity.⁴⁹ Li et al. used CuO (obtained using an easy, single-step hydrothermal approach) to activate PMS for removing acid orange AO7 dye. CuO displayed excellent catalytic activity (95.38% of dye removed after 15 min) with 5 mM PMS. This indicates that CuO and PMS are good candidates for the degradation of very stable, toxic POPs (e.g. acid orange AO7) through the formation of ·OH and ˙SO₄⁻ radicals.⁵⁰

Researchers have also combined metal ions (Mn⁺) to activate PMS with electrocatalysis to create an electrocatalysis/Mn⁺/PMS system that enhances the drug removal efficiency in a short time, thus decreasing the consumption of electrical energy.⁵¹

In our work, we have made significant advancements in the formation of pores within CCTO, aiming to enhance the active sites available for catalysis. Additionally, we have increased the amount of the CuO phase, a step designed to effectively enhance the activation of sulfate radicals. The electrodes were prepared by pressing the CCTO powder with different amounts of polymethyl methacrylate (PMMA) using a hydraulic press. PMMA promoted pore formation and oxidation of the ex-soluted Cu₂O phase into CuO. After studying the morphology, structure, and optical characteristics of the obtained samples, paracetamol degradation in the presence of PMS was monitored to determine their electrocatalysis efficiency. Lastly, the radicals implicated in paracetamol degradation, the total organic carbon (TOC), and toxicity were determined.

2. Experimental section

2.1. Chemicals

Titanium(IV) oxide (TiO₂, 99.5%, CAS number: 13463-67-7), calcium carbonate (CaCO₃, 98%, CAS number: 471-34-1), PMMA (CAS number: 9011-14-7), potassium hydroxide (KOH, ≥85%, CAS number: 01900-20-08), poly(vinyl alcohol) (PVA, 99%, CAS number: 9002-89-5), paracetamol (PCM, ≥99% CAS number: 103-90-2), potassium chloride (KCl, ≥99.0%, CAS number: 7447-40-7), sodium chloride (NaCl, ≥99%, CAS number: 7647-14-5), sodium sulfate (Na₂SO₄, ≥99%, CAS number: 7757-82-6), PMS (CAS number: 70693-62-8), methanol (CH₃OH, ≥99.9%, CAS number: 67-56-1) tert-butanol ((CH₃)₃COH, ≥99.5%, CAS number: 75-65-0), and p-benzoquinone (C₆H₄O₂, ≥99.5%, CAS number: 106-51-4) were purchased from Sigma Aldrich. Copper(II) oxide (CuO, CAS number: 1317-38-0, 98%) was from Alfa Aesar. All chemicals were used without any further purification.

2.2. Preparation of CCTO porous membranes

After mixing CaO, CuO, and TiO₂ (1 : 3 : 4 mol%), as powders, in a planetary ball mill (PM200, Retsch) for 5 h (rotation speed = 350 rpm, alumina balls (powder-to-ball mass ratio of 1 : 9) in an alumina jar) and the powder was calcined at 900 °C for 3 h. Then, the powder was mixed (rotation speed = 250 rpm) with different amounts of the pore-forming agent PMMA (20, 30 and 40 wt%) for 1 h to obtain a homogeneous mixture. After milling, 5 wt% PVA was added, and the powder was pressed into pellet disks at about 9 T using a hydraulic press. Pellets (20 mm in diameter and 1–2 mm thick) were sintered in air for 3 h (2 °C min⁻¹ up to 240 °C, followed by 1 °C min⁻¹ up to 420 °C and 2 °C min⁻¹ up to 850 °C to ensure complete PMMA burning, and then 5 °C min⁻¹ up to 1100 °C). Four pellets were prepared: CCTO without PMMA (CCTO), and CCTO-20, CCTO-30 and CCTO-40 with 20, 30, and 40 wt% PMMA, respectively.

2.3. Characterization of the synthesized nanocomposites

The pellet's surface morphology, crystallinity, and composition were analyzed using scanning electron microscopy (SEM), three-dimensional (3D) optical microscopy, energy-dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD) analysis with Rietveld refinement, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), mercury porosimetry, and electron paramagnetic resonance (EPR). The optical properties were studied using photoluminescence. Further details about the parameters and instruments can be found in the ESI† Part S1.

2.4. Electrochemical properties

For these measurements, a Solartron SI 1287 potentiostat/galvanostat, a three-electrode system, and 1 M KOH (electrolyte) were used. The samples (diameter = 2 cm), Ag/AgCl, and platinum wire were the working electrode, reference electrode, and counter electrode, respectively. Before all experiments, electrolytes were degassed with pure argon for 30 minutes to eliminate the oxygen in the solution. The sample oxidation and reduction potentials were determined by cyclic voltammetry. The interfacial charge transfer resistance of the electrode was measured by electrochemical impedance spectrometry (EIS): frequency from 0.01 Hz to 105 MHz and voltage bias of 10 mV amplitude.

2.5. Electrocatalytic system for PCM degradation

PCM was chosen as a target pollutant to assess the catalytic removal effectiveness of the prepared electrodes. PCM (initial concentration of 10 ppm), 0.5 mM PMS (active component), and 50 mM Na₂SO₄ (electrolyte solution) were added into a beaker to reach a total volume of 210 ml. The three-electrode system described in section 2.4 was used for all experiments. Before electrolysis, the solution was degassed by 30 minutes of argon bubbling to eliminate dissolved oxygen. During the entire experiment (240 minutes), the solution was stirred, and at different time-points, 1 mL aliquots were collected using a disposable syringe and filtered.

2.6. Analytical procedures

PCM concentration in the different aliquots was determined by high-performance liquid chromatography (HPLC) coupled with mass spectrometry (MS) as previously described in ref. 22 and 52. PCM elimination efficiency was then determined using the HPLC-MS data and eqn (1):⁵³where C₀ (mg L⁻¹) is the PCM concentration at time = 0, and C (mg L⁻¹) is the PCM concentration in the solution at the different time-points during electrocatalysis.

2.7. Scavenger studies

The reactive species involved in the electrocatalysis system during PCM mineralization were identified with trapping experiments by adding the following scavengers into the electrocatalysis solution: 6.6 mM p-benzoquinone, 660 mM tert-butyl alcohol, and 660 mM methanol to trap superoxide (˙O₂⁻), hydroxyl ˙OH, and sulfate (˙SO₄⁻) and ˙OH radicals, respectively.

2.8. Toxicity tests

The acute toxicity of PCM and its intermediate by-products was assessed with a bioluminescence inhibition assay in which the bioluminescence changes⁵⁴ of the Vibrio fischeri LCK 487 strain are measured and analyzed using the Microtox® Model 500 analyzer (Modern Water Inc.; UK) and the MicrotoxOmni® software. The activation of V. fischeri is described in the ESI† Part S2. Eqn (2)⁵⁵ was used to calculate V. fischeri luminescence inhibition rate:where LU(t) is V. fischeri luminescence intensity after 5 min of contact with the sample, LU(0) is the initial luminescence (without sample), and R(t) is the correction term. As V. fischeri luminescence decreases naturally over time even in a control solution (MilliQ water and NaCl), R(t) is calculated with eqn (3):²where LU0(t) is V. fischeri luminesce intensity after 5 min in the control solution and LU0(0) is the initial luminescence intensity before addition of the control solution.

2.9. TOC and energy consumption

PCM degradation was assessed by measuring the TOC of the initial and treated samples using a TOC-L CSH/CSN Shimadzu (Japan) analyzer. A TOC standard solution of 1000 mg L⁻¹ was used to generate the calibration curves. TOC removal rate was calculated with eqn (4):⁵⁶where Δ(TOC)_exp is the TOC decrease (mg L⁻¹) at the different time-points, and TOC₀ is the baseline TOC value.

The energy consumption per TOC mass unit (EC_TOC) was determined with eqn (5):

where U is the applied voltage (in V), I the current (in A), t the electrolysis time (in h), Δ(TOC)_exp the experimental TOC decrease (g L⁻¹), and V_s the solution volume (in L).

3. Results and discussion

3.1. Characterization of the synthesized composites

The CCTO powder was mixed or not with different PMMA amounts, and after milling the pressed pellet disks were sintered (as described in Fig. S1†) to fully eliminate PMMA and to obtain porous pellets.

SEM analysis (Fig. 1) clearly indicated that all pellets had a similar morphology with the presence of two main structures: grains and grain boundary regions (between the largest grains). The CCTO-20, CCTO-30 and CCTO-40 samples presented additional voids that confirmed the formation of porous ceramics upon PMMA addition. Mercury porosimetry confirmed that CCTO porosity increased upon PMMA addition from 5.4% (0% PMMA) to ∼50% in all samples, regardless of the PMMA amount used (20%–30%–40%) (Table 1). Additionally, the pore size distribution, as illustrated in Fig. 1b, demonstrates a concurrent increase in pore size with the introduction of PMMA. Specifically, the CCTO-20 sample exhibited a pore size of 39.9 μm. However, with the incorporation of 40% PMMA, the pore size in the membrane expanded to 55.1 μm (Table 1). This increase can be attributed to the agglomeration of PMMA within the membrane structure.

Fig. 1 (a) SEM images of the CCTO samples prepared using different amounts of PMMA (0, 20, 30, and 40 wt%) and (b) pore size distribution.

Table 1 Porosity and pore size of the CCTO samples prepared using different PMMA amounts (0, 20, 30, and 40 wt%)

Sample	Porosity (%)	Pore size (μm)
CCTO	5.4 ± 2.9	—
CCTO-20	48.0 ± 2.5	39.9 ± 2.8
CCTO-30	49.9 ± 1.6	44.7 ± 3.6
CCTO-40	51.7 ± 3.7	55.1 ± 2.1

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These results were confirmed by 3D optical microscopy that showed a homogeneous pore distribution at the surface of the CCTO-20, CCTO-30 and CCTO-40 samples (Fig. S2†). The chemical composition and element distribution (Fig. 2), determined by EDX, revealed that Ca and Ti were present in grains, Cu in the grain boundaries, and O in both. This suggests that the CuO phase was in the boundary regions and the CCTO phase in the grains.

Fig. 2 SEM/EDX mapping of the CCTO samples prepared using different PMMA amounts (0, 20, 30, and 40 wt%).

The main XRD patterns of the different samples after sintering could be attributed to the cubic perovskite structure of CCTO (the main crystalline phase), and the other low-intensity XRD reflections were assigned to a secondary CuO phase (Fig. 3a). PMMA addition increased the CuO phase intensity. However, in the XRD patterns of powdered samples, obtained by grinding the sintered pellets, another minor phase of Cu₂O appeared in CCTO and CCTO-40, suggesting that this phase was present mainly in the bulk of the materials (Fig. 3b). No XRD reflection corresponding to the carbon or carbide-based crystalline phase was observed, confirming PMMA thermal decomposition into volatile gaseous species upon sintering. Then, Rietveld analysis of the XRD data of the powdered samples (Cu₂O was observed only in the powdered samples) showed that in all plots (Fig. S3†), the main phase was CCTO with a minor CuO phase. The weight fraction of the CuO phase progressively increased and that of the CCTO phase decreased in samples with increasing PMMA amount (Fig. 3c). CuO phase formation, related to the eutectic point of CuO–TiO₂ (liquid phase), occurred at the sample surface. At this point, atoms can migrate and diffuse easily between grains and grain boundaries. Moreover, it has been reported that Cu²⁺ is not stable at high temperature and tends to be reduced to Cu⁺. Consequently, Ti⁴⁺ may be substituted at Cu sites for charge compensation or a Cu₂O phase may be formed.⁵⁷ Upon cooling, Cu⁺ might donate electrons to the Ti 3d conduction band and convert back to Cu²⁺, forming a CuO phase.⁵⁷ The CuO phase in grain boundaries might be explained by CuO migration into such regions during the cooling, as reported in our previous studies.^38,40 The CCTO sample also contained 3.2 wt% Cu₂O, presumably due to the limited oxygen diffusion from the surface to the bulk in the absence of a porous morphology. However, when PMMA was added to CCTO, CuO was mainly detected and only negligible amounts of Cu₂O, suggesting the Cu₂O side phase oxidation into CuO due to enhanced oxygen diffusion in the pores formed upon PMMA addition. In CCTO-40, 1.0 wt% of Cu₂O still remained in the sample, possibly due to PMMA agglomeration when at higher concentrations. In this sample, PMMA agglomeration might limit Cu₂O oxidation. Furthermore, higher PMMA concentrations led to increased carbon in the sample that can react with oxygen and limit the diffusion into the bulk.

Fig. 3 (a) XRD patterns of CCTO pellets prepared using different amounts of PMMA (0, 20, 30, and 40 wt%). (b) XRD patterns of the same samples after powdering. (c) Weight fraction of the formed crystalline phases (Rietveld analysis of the XRD data for the powdered samples).

Raman spectroscopy (Fig. 4a) clearly showed two strong bands at 445 cm⁻¹ and 510 cm⁻¹, and a weaker band at 291 cm⁻¹. The 445 cm⁻¹ band was assigned to A_g(1), and the 512 cm⁻¹ band to A_g(2). These are pseudo-modes that originated in the [TiO₆] cluster oscillation within the CCTO structure. The 291 cm⁻¹ band was assigned to the CuO mode.⁵⁸ Raman mapping was used to confirm the elements present in the grains and grain boundaries. The grains (orange) and grain boundary regions (black) were perfectly separated in the Raman image at 510 cm⁻¹ (Fig. 4b), and the peak intensity corresponded to the A_g(2) mode of CCTO. The peak at 291 cm⁻¹ (Fig. 4c) was assigned to the Raman active mode of CuO, which is shown in black in the grains and in orange in the grain boundaries.⁵⁹ These results confirmed the SEM/EDX results, which suggested that CuO is present at the grain boundaries and CCTO in the grains. The Raman peak shift was explained by the CuO phase formation in grain boundaries and the CCTO phase deformation.⁶⁰ To obtain more details on the chemical compositions and chemical states, the different samples were characterized by XPS. Cu, Ti and O with different chemical states were detected in all samples. Fig. 4d shows the high-resolution Cu 2p spectra and the Cu 2p_3/2 (934.5 eV) and Cu 2p_1/2 (954.2 eV) peaks.^59,61,62 After deconvolution, in the CCTO spectrum, the Cu 2p_3/2 and Cu 2p_1/2 peaks were each composed of two peaks. The two main peaks (954 eV and 932 eV) were assigned to Cu²⁺, and the two small shoulder peaks (951.5 eV and 931.5 eV) to Cu¹⁺, as expected for Cu₂O. Shake-up satellites (937 to 945 eV), typical of Cu²⁺, also were present. In CCTO-20 and CCTO-30, prepared with 20 and 30 wt% PMMA, respectively, the peaks related to Cu¹⁺ were not detected. This confirmed the Rietveld refinement results, suggesting that the Cu₂O phase was oxidized to CuO at high temperatures due to the formation of pores that facilitate oxygen diffusion.^62,63 In CCTO-40 (40% PMMA), the shoulder peaks of Cu¹⁺ reappeared, in agreement with the XRD measurements showing the Cu₂O phase in this sample. Fig. 4e shows the high-resolution Ti 2p spectra and the Ti 2p_3/2 (458.8 eV) and Ti 2p_1/2 (463.3 eV) peaks. After deconvolution, this spectrum included four peaks: two peaks at 457.8 eV and 463.9 eV (assigned to Ti⁴⁺) and two peaks at 458.5 eV and 464.3 eV (Ti³⁺). The binding energies at 459.1 eV were assigned to TiO₅ clusters.^64,65 The Ti 2p peaks were slightly shifted to higher binding energy (+0.2 eV and +0.3 eV) upon PMMA addition due to the formation of the CuO phase and of oxygen vacancies in the TiO₂ lattice that promote electron attraction.²¹ The O 1s spectra after deconvolution (Fig. 4f) included peaks at 529.43 eV (assigned to the Cu–O bond), at 530 eV (Ti–O bond), and at 532.08 eV (surface adsorbed oxygen, i.e. adsorbed water).^38,40

Fig. 4 (a) Raman shifts of the CCTO samples prepared using different PMMA amounts (0, 20, 30, and 40 wt%). (b and c) Raman maps: (b) 510 cm⁻¹ (CCTO) and (c) 291 cm⁻¹ (CuO). (d–f) High-resolution XPS spectra of (d) Cu 2p_3/2, (e) Ti 2p_3/2, and 9f) O 1s for the CCTO samples prepared using different PMMA amounts (0, 20, 30, and 40 wt%).

The electronic and structural characteristics of the samples were correlated using photoluminescence measurements. Fig. 5a shows the photoluminescence curves of the CCTO, CCTO-20, CCTO-30, and CCTO-40 samples at room temperature. All samples had a peak with a maximum intensity at ∼370 nm, attributed to valence band transitions within TiO₆ clusters.^66,67 In the emission spectra, the shoulder at 375 nm was explained by shallow defects near the conductance band.⁶⁷ It has been reported that TiO₆ clusters are responsible for photoluminescence in CCTO, whereas formation of TiO₅ clusters leads to photoluminescence quenching.⁶⁸ The change in the balance between TiO₆ and TiO₅ might explain the observed photoluminescence quenching.⁶⁸ All samples displayed similar spectral shapes, suggesting that all materials had a very similar electronic band structure.⁴⁰ In the samples with PMMA, the overall photoluminescence intensity decrease, without shifts in peak positions, suggests an increase in the concentration of specific surface defects in the CCTO material that might promote separation of the photogenerated charges.^66,69,70 The intensity decrease was most pronounced in the CCTO-30 sample.

Fig. 5 (a) Photoluminescence spectra and (b) room-temperature cw EPR spectra of the CCTO-20, CCTO-30, CCTO-40 and CCTO samples.

The EPR spectra (Fig. 5b) of all samples showed a strong signal of comparable (double-integrated) intensity at g = ∼2.15, which was previously attributed to Cu²⁺ species.⁷¹ This finding suggests that the amount of Cu²⁺ species was similar in all studied samples. Yet, in the CCTO-30 sample, the amplitude was clearly smaller (by a factor of ∼2) and the ΔB_pp linewidth larger compared with the other samples (∼5 mT versus ∼3.3 mT). This is indicative of altered spin-interactions of the detected Cu²⁺ species. Like for the decreased photoluminescence intensity of the CCTO-30 sample, this change may be linked to a modification of defects in CCTO.

The electrochemical properties of the CCTO pellets with/without different amounts of PMMA were investigated using a three-electrode system in 1 M KOH. In the cyclic voltammetry curves (scan rate of 25 mV s⁻¹ in the potential range of 0–1 V vs. Ag/AgCl) (Fig. 6a), CuO oxidation and reduction peaks were detected. CCTO displayed the lowest intensity compared with the porous CCTO samples prepared using PMMA. This can be attributed to the better contact and wettability of the porous CCTO-20, CCTO-30 and CCTO-40 samples in the electrolyte solution.^62,72 Moreover, CCTO-30 had the highest current density (0.82 mA cm⁻²versus 0.63 and 0.71 mA cm⁻² for CCTO-40 and CCTO-20, respectively). This result may be related to the higher weight fraction of the CuO phase in the CCTO-30 sample (verified by the Rietveld refinement analysis of XRD data) and the formation of defects (indicated by the photoluminescence and EPR results), such as oxygen vacancies, that expose more active surface sites.^73,74 The cyclic voltammetry data were in agreement with the EIS results (Fig. 6b). Data were fitted with the Z-View software and the equivalent electrical circuit R + Q_CPE//R_ct (R = electrolyte resistance, Q_CPE = constant phase element, and R_ct = charge transfer resistance). The EIS plot shows that CCTO displayed the highest arc radius compared with the CCTO samples with PMMA. This indicates an increase in the charge transfer capacity following the formation of pores and the increase in surface contact. Comparison of the resistance values of CCTO, CCTO-20, CCTO-30, and CCTO-40 indicated that CCTO-30 had the fastest interfacial charge migration. This may be related to the increase in the CuO phase and in the defect amount, presumably oxygen vacancies, that facilitate charge transfer in CCTO-30.^75,76

Fig. 6 (a) Cyclic voltammetry curves obtained using CCTO, CCTO-20, CCTO-30 and CCTO-40 as electrodes in 1 M KOH (scan rate = 25 mV s⁻¹). (b) Nyquist plots showing the electrochemical impedance spectroscopy data for CCTO, CCTO-20, CCTO-30 and CCTO-40.

The electrocatalytic activity of CCTO samples with/without PMMA as anodes was evaluated by following the degradation of 10 ppm PCM in 210 mL of sodium sulfate (electrolyte solution) with 0.5 mM of PMS (active component) at the fixed potential of 1.5 V vs. Ag/AgCl (Fig. S4†). PCM degradation was limited to 64% with CCTO, even after 4 h. Conversely, with the porous CCTO samples prepared using different PMMA amounts, PCM was completely removed after 90 min (CCTO-30), 150 min (CCTO-40), and 210 min (CCTO-20) (Fig. 7a and b). Thus, PMMA addition clearly improved PCM removal efficiency. CCTO-30 was the most effective and faster anode for PCM degradation. This could be explained by the high charge transfer (EIS data) linked to the increase in defects, and the CuO phase amount. The generation of oxygen vacancies may allow producing more active defects, thus enhancing the electrochemical degradation of pollutants.^77,78 Furthermore, based on literature data, the CuO phase promotes PMS activation to produce SO₄⁻ free radicals, according to eqn (6).⁴⁸ In addition, the Cu(II)–HSO₅ combination might yield both Cu(III) and ·OH by an alternative mechanism of one-electron transfer (eqn (7)). It has been shown that these radical Cu(III) species act as a selective oxidant (eqn (8)) for PCM removal due to their reducing potential (1.57–2.3 V).^79,80

Cu²⁺ + HSO₅⁻ → Cu¹⁺ + SO₄˙⁻ + OH⁻ (6)

Cu²⁺ + HSO₅⁻ → Cu³⁺ + SO₄²⁻ + OH˙ (7)

Cu³⁺ + organic pollutants → Cu²⁺ + oxidation products (8)

SO₄⁻/˙OH + organic pollutants → SO₄²⁻/H₂O + oxidation products (9)

e⁻ + O₂ → ˙O₂⁻ (10)

Fig. 7 (a) PCM degradation by electrocatalysis via PMS activation. (b) PCM removal efficiency. (c) Radical scavengers test. TBA: tert-butanol (·OH scavenger); MeOH: methanol (·OH and ˙SO scavenger); BQ: p-benzoquinone (˙O₂⁻ scavenger).

To better assess the role of oxidizing radicals in PCM degradation, the degradation of 10 ppm PCM in 210 mL of sodium sulfate electrolyte solution with 0.5 mM PMS was monitored in the presence of radical scavengers. PCM degradation strongly decreased, from 96% to 70%, in the presence of tert-butanol, a well-known ·OH scavenger^22,81 (Fig. 7c). PCM degradation was reduced to 20% when methanol (a scavenger for ·OH and ˙SO⁴⁻)⁸² was used, and to 2% with p-benzoquinone (a scavenger of ˙O₂⁻).⁵² This suggests that ˙O₂⁻ radicals play a key role in PCM degradation in the presence of PMS. Overall, this study revealed that ˙SO₄⁻ and ˙O₂⁻ radicals were the main reactive species, whereas ·OH radicals were only a minor species in PCM degradation.

The stability of the CCTO-30 anode was monitored during five successive cycles (anode rinsed in deionized water several times after each cycle). CCTO-30 pellets remained stable without losing activity after five cycles, demonstrating their potential as a reusable electrocatalyst (Fig. 8a).

Fig. 8 (a) Reusability of CCTO-30 over five successive cycles. (b) PCM removal using different systems. (c) TOC removal during PCM degradation (EC–PMS–CCTO-30 system). (d) Vibrio fischeri luminescence inhibition during PCM degradation.

To evaluate the effect of PMS on the electrocatalytic process, different systems were tested for PCM degradation. No PCM degradation was observed in the systems with only CCTO and with only PMS (Fig. 8b) at room temperature.⁸³ In the electrocatalysis (EC)–CCTO-30 binary system, only 6.8% of PCM was degraded, possibly because not enough radicals are produced to completely degrade organic molecules. Even in the PMS–CCTO-30 system, only 8.5% of PCM was removed, confirming the need of anodic oxidation.⁸⁴ Conversely, when electrocatalysis and PMS–CCTO-30 were coupled (EC–PMS–CCTO-30), PMS was efficiently activated and 100% of PCM was degraded in 90 min. This improvement was also the result of PMS activation forced by the electron transfer occurring at the anode surface.⁸⁵ The much higher PCM degradation obtained with the EC–PMS–CCTO-30 system (100.0%), compared with the EC–CCTO-30 (6.8%) and PMS–CCTO-30 (8.5%) systems, indicates the existence of a distinct, synergistic effect between electrocatalysis and PMS–CCTO-30.⁸⁶

TOC content was quantified to determine the degree of PCM degradation using CCTO-30 as anode. TOC rapidly decreased in the first 60 minutes (Fig. 8c).^87,88 The TOC removal rate of 42.7 ± 2.1% at 4 h suggested PCM degradation into other smaller organic compounds. At longer time points, mineralization gradually increased due to the development of degradation by-products (e.g. short-chain carboxylic acids and aliphatic organic acids, such as fumaric, oxalic, acetic, and maleic acids) with slower radical reaction kinetics.^2,89 Indeed, TOC was still not completely eliminated after 12 h of reaction.^56,90

As the aim of wastewater treatments is to reduce harmful substances, it was important to study the toxicity of PCM and also of these smaller chain aliphatic organic acids that persisted in solution. First, V. fischeri bioluminescence inhibition was 74% upon PCM addiction (Fig. 8d). After 1 h of PCM degradation, inhibition increased to 79%, suggesting the formation of even more hazardous by-products, for instance p-benzoquinone, benzaldehyde and benzoic acid.^2,85,90 After 2 h of electrocatalysis, bioluminescence intensity inhibition decreased to ∼9%, indicating the decomposition of these hazardous intermediate by-products to less harmful chemicals.⁹¹ After 10 h, luminescence inhibition was very low, suggesting a complete transformation of PCM and its by-products into non-toxic compounds.²¹

The energy consumption per TOC was 1.45 kW h g_TOC⁻¹. This is a relatively low energy consumption compared with previous studies on PCM mineralization (e.g. 3.6 kW h g_TOC⁻¹ for PCM mineralization using electro-Fenton).²

In recent years, perovskite materials have attracted much attention as highly active anodes for wastewater treatment by electrocatalytic oxidation. As the CCTO perovskite used in this work has not been studied before as an anode in electrocatalytic water remediation, its performance was compared with that of other perovskite catalysts (summary in Table 2). Although the direct comparison was hampered by differences in decomposition set-ups, the CCTO-30 electrode showed high degradation efficiency in a short time (90 min) and with low electricity consumption, despite a similar small surface area per gram (3.14 cm²), compared with other perovskite anodes. This indicates that this CCTO material could be a promising anode for electrochemical wastewater treatment.

Table 2 Comparison of studies on the use of perovskite anodes for the electrocatalytic degradation of organic pollutants

Pollutant	C pollutant (mg L^−1)	Anode material	Current or voltage	Solution volume (mL)	pH	Degradation time	Removal efficiency (%)	Ref.
Methyl red	140	Ti/CoTiO3/Ce–PbO2	30 mA cm^−2	100	7	180	90	92
Tetracycline hydrochloride	20	CuxCo1−xMn2O4/Ti (14.24 cm^2)	1.70 V	250	3	120	91.3	37
Methyl orange	20	SrxLa1−xMnyCo1−yO3−δ (16 cm^2)	20 mA cm^−2	200	9	60	99.61	93
Ciprofloxacin	5	BiFeO3 with graphite nanoparticles	1.5 V	75	6.8	240	36	94
Paracetamol	10	CCTO-30 (3.14 cm^2)	1.5 V	210	7	90	100	This work

---

4. Conclusion

In summary, porous calcium copper titanate (CCTO) pellets were synthesized by adding polymethyl methacrylate (PMMA) as a pore-forming agent to CCTO powder before ball milling, pressing, and thermal treatment under air atmosphere. 3D microscopy images confirmed the homogenous pore distribution in the obtained ceramic pellets due to PMMA thermal decomposition, yielding porosities of ∼50%. Electrochemical measurements revealed that CCTO-30 (with 30% PMMA) contained the highest amount of CuO phase and displayed the lowest resistance, the highest charge transfer capacity, and the highest redox peaks. Then, all samples were tested as anodes for paracetamol (PCM) degradation via peroxymonosulfate (PMS) activation. Total PCM removal was achieved by the CCTO-30 electrode after 90 min. Moreover, this sample remained stable without loss of activity after five cycles. The scavenging test experiments revealed the major role of ·SO₄⁻ and ·O₂⁻ radicals and the minor role of ·OH radicals in PCM degradation. Thus, the fabrication of porous CCTO electrodes using PMMA is a suitable approach for the development of electrocatalysis materials in the advanced oxidation processes (AOPs) field to remove persistent organic pollutants (POPs) from wastewater.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This project has received funding from the Research Council of Lithuania (LMTLT), the GILIBERT 2021 program agreement No S-LZ-21-4 and was co-founded by Campus France grant No. 46593RA (PHC GILIBERT 2021).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3en00317e

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