S, N, P, and B-doped bio-carbons as dual catalysts for tetracycline degradation via the heterogeneous electro-Fenton process
Abdelhakim Elmouwahidi, Edgar Fajardo-Puerto, María Pérez-Cadenas, Esther Bailón-García, Agustín F. Pérez-Cadenas, Francisco Carrasco-Marín
TL;DR
This paper explores using bio-carbon doped with heteroatoms as a dual catalyst for environmental cleanup, showing improved performance in degrading tetracycline in wastewater.
Contribution
The novelty lies in the use of doped bio-carbon as a metal-free dual catalyst for both oxygen reduction and tetracycline degradation.
Findings
Nitrogen-doped bio-carbon showed the highest oxygen reduction reaction activity with a JK value of 10.4 mA cm⁻2.
All doped samples achieved tetracycline degradation efficiencies over 40%, with nitrogen-doped reaching up to 70%.
The study demonstrates a cost-effective method for producing catalysts from agro-industrial waste for environmental remediation.
Abstract
Bio-carbon (BC) derived from agro-industrial waste was doped with various heteroatoms (S, N, B, P) to evaluate its catalytic performance in the oxygen reduction reaction (ORR) and the electro-Fenton process. The materials were thoroughly characterized using techniques such as nitrogen adsorption at 77 K, carbon dioxide adsorption at −193 K, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Electrochemical performance was assessed via cyclic voltammetry (CV) and linear sweep voltammetry (LSV). All doped samples (S, N, B, P) showed improved ORR activity for hydrogen peroxide generation, with the nitrogen-doped bio-carbon (DBC-N) exhibiting the highest performance. This was attributed to its superior JK value (10.4 mA cm⁻2) and the lowest onset potential (E° onset = −0.14 V). Beyond their ORR catalytic activity, the doped samples were also tested as dual-function…
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Taxonomy
TopicsAdvanced oxidation water treatment · Environmental remediation with nanomaterials · Microbial Fuel Cells and Bioremediation
Introduction
Emerging contaminants—including cleaning agents, disinfectants, and pharmaceutical residues—have recently garnered increasing attention due to their potential risks to public health and environmental integrity (Munoz et al. 2016). Among these, antibiotics are of particular concern, as their persistence in aquatic and terrestrial ecosystems contributes to the proliferation of antimicrobial resistance genes, posing serious threats to human health, food safety, and water quality (Geng et al. 2022; Qian et al. 2023; Zhang et al. 2022a, b, c). Notably, a substantial proportion of administered antibiotics (10–70%) are excreted unmetabolized, re-entering the environment through wastewater and agricultural runoff (Gao et al. 2022; Xie et al. 2023). Projections indicate that global antibiotic consumption may increase by 15% to 200% by 2030, further exacerbating disposal and contamination challenges (Liu et al. 2022a, b; Song et al. 2022).
Tetracycline (TC), a broad-spectrum antibiotic widely used in both human and veterinary medicine, has become a model compound for evaluating advanced treatment technologies (Sadeghzadeh et al. 2025; Rashid et al. 2023). Electrochemical advanced oxidation processes (EAOPs) have emerged as promising strategies for the degradation of persistent organic pollutants, offering high mineralization efficiencies (Chi et al. 2024). Among these, the electro-Fenton (EF) process has demonstrated effective degradation of various antibiotics—including sulfadiazine, ciprofloxacin, cefalexin, oxcarbazepine, prednisolone, chloramphenicol, and thiamphenicol—with TC being particularly responsive (Li et al. 2020; Moratalla et al. 2021; Zhang et al. 2020).
The EF process operates through the in situ generation of hydroxyl radicals (•OH), which are produced via the reaction between electrogenerated hydrogen peroxide (H₂O₂), formed through the two-electron oxygen reduction reaction (ORR) (Eq. 1), and ferrous ions (Fe^2^⁺) acting as the Fenton catalyst (Eq. 2), typically under acidic conditions (Liu et al. 2021; Puga et al. 2020; Xin et al. 2022; Xing et al. 2022). This mechanism underscores the importance of developing efficient, metal-free electrocatalysts capable of promoting H₂O₂ generation and activation for environmental remediation applications. In this regard, recent advances in heterogeneous Fenton-like processes highlight that the design of advanced catalyst is fundamental to maximizing the production of reactive oxygen species and ensuring the efficient degradation of complex organic pollutants (Qaretapeh et al. 2024).
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm O}_2\;+\;2\mathrm H^+\;+\;2\mathrm e^-\;\rightarrow\;{\mathrm H}_2{\mathrm O}_2$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm{Fe}2^++\;{\mathrm H}_2{\mathrm O}_2\;+\;\mathrm H^+\;\rightarrow\;\mathrm{Fe}^{3+}\;+\;{\mathrm H}_2\mathrm O\;+\;{}^\bullet\mathrm{OH}$$\end{document}The electro-generation of H₂O₂ is widely recognized as the rate-limiting step in the electro-Fenton process (Yu et al. 2019), prompting extensive research into oxygen reduction reaction (ORR) catalysts with high selectivity toward the two-electron pathway. While platinum (Pt) remains the benchmark catalyst for ORR due to its excellent activity, its high cost and scarcity significantly limit its practical application in large-scale or decentralized systems (Thor et al. 2022).
Carbon-based materials—such as graphene oxide, graphite felt, reduced graphene oxide, and carbon felt—have emerged as promising alternatives owing to their low cost, chemical stability, and favorable selectivity for H₂O₂ production via the two-electron ORR pathway (Cheng et al. 2022; Liu et al. 2022a, b; Zhang et al. 2022a, b, c). However, their intrinsic ORR activity remains inferior to that of Pt, necessitating further modification to enhance catalytic performance.
Heteroatom doping has proven to be an effective strategy for improving the electrocatalytic properties of carbon materials. Dopants such as oxygen (O), nitrogen (N), phosphorus (P), boron (B), and fluorine (F) can modulate the electronic structure by altering surface charge distribution, thereby enhancing electron transfer and creating additional active sites for ORR (Chen et al. 2021; Han et al. 2022; Li et al. 2023). In this context, the development of carbon-based nanostructures, such as reduced graphene oxide derived from biomass, has shown significant potential in improving charge transfer and providing a higher density of active sites for the degradation of persistent organic pollutants (Irani et al. 2024). Moreover, the presence of mesopores has been shown to facilitate oxygen diffusion, further improving ORR kinetics. Recent studies have demonstrated that alkali activation of carbon precursors promotes mesopore formation, contributing to improved mass transport and catalytic efficiency (Shen et al. 2022). Furthermore, the use of sustainable carbon frameworks derived from biomass allows for the synergistic distribution of these active sites, which is essential for maximizing the performance of metal-free electrocatalysts in environmental applications (Nalkyashree et al. 2024).
In summary, carbon-based materials with tailored surface chemistry, hierarchical porosity, and enhanced conductivity represent a promising class of metal-free electrocatalysts for both ORR and electro-Fenton applications, particularly in the context of sustainable water treatment and energy conversion technologies.
In this study, bio-carbon was synthesized from agro-industrial waste originating from the olive oil production process, specifically from alperujo. Alperujo is the semi-solid residue composed of aqueous and fibrous plant material remaining after the mechanical extraction of olive oil (Dauber et al. 2022). Currently, alperujo is managed through various pathways, including energy recovery (combustion of the residual solid fraction), soil amendment, and composting. However, these methods face significant hurdles. Direct soil application is limited by the high concentration of phytotoxic polyphenols and the risk of long-term soil salinization (García-Randez et al. 2023; Laribi et al. 2025; Moreno-Maroto et al. 2019). Regarding energy production, the high moisture content of raw alperujo (typically > 60%) makes the drying process prior to combustion extremely energy-intensive and technically challenging (Roig et al. 2006). Furthermore, its complex organic load and unbalanced C/N ratio pose difficulties for efficient biological stabilization through composting (Chowdhury et al. 2013). In this context, due to its phytotoxicity and low biodegradability, alperujo is considered one of the most environmentally hazardous effluents in the agri-food sector (Quintana-Gómez et al. 2021). To address this issue, the present work embraces a circular economic approach by valorizing alperujo through its transformation into functional carbon materials. This strategy not only mitigates waste accumulation but also promotes resource efficiency and environmental sustainability.
The novelty of this research resides in the development of a sustainable, metal-free bifunctional system for the heterogeneous electro-Fenton process. By tailoring the surface chemistry through heteroatom doping of alperujo-derived bio-carbon, it is hypothesized that these materials could act as dual-purpose catalysts, potentially facilitating both the in situ production of H_2_O_2_ and its subsequent activation into reactive oxygen species, such as •OH. This proposed mechanism would eliminate the need for external Fenton reagents and provide a sustainable solution for the degradation of persistent antibiotics, offering a significant advancement in wastewater treatment technologies based on the valorization of agro-industrial byproducts.
The bio-carbon produced was subsequently doped with heteroatoms—namely sulfur (S), nitrogen (N), boron (B), and phosphorus (P) via hydrothermal treatment. The resulting materials were evaluated for their electrocatalytic performance in the oxygen reduction reaction (ORR) and their efficiency in the electro-Fenton (EF) process for the degradation of tetracycline (TC), a model emerging contaminant.
Materials and methods
Preparation of carbon materials
The synthesis of bio-carbon (BC) was carried out using a method adapted from previous research (Elmouwahidi et al. 2017). In the initial step, olive wastewater—pre-dried to remove moisture—was subjected to pyrolysis at 300 °C for 2 h under a continuous nitrogen atmosphere (60 cm^3^ min⁻^1^). The resulting char was then chemically activated by blending it with potassium hydroxide (KOH) in a 1:1 weight ratio. This mixture underwent a second thermal treatment at 840 °C for 2 h, again under nitrogen flow, yielding the final BC material.
To introduce heteroatom functionalities, BC was doped with nitrogen (N), boron (B), phosphorus (P), and sulfur (S) using specific molecular precursors: melamine (100 mg), phenylboronic acid (600 mg), triphenylphosphine (1 g), and 1-dodecanethiol (8 mL), respectively. For N and B doping, 1.5 g of BC was dispersed in 100 mL of deionized water. For P and S doping, the dispersion medium consisted of a 1:1 mixture of ethanol and deionized water (50 mL each). Each suspension was subjected to hydrothermal treatment at 130 °C for 24 h. The doped materials were labeled as DBC-X, where X denotes the incorporated heteroatom.
Textural and chemical characterization
Nitrogen (N₂) and carbon dioxide (CO₂) adsorption isotherms were recorded using a Quadrasorb analyzer (Quantachrome Instruments) to evaluate the surface area and porosity of the samples. Prior to analysis, all materials were degassed under high vacuum (10⁻⁶ mbar) at 120 °C for 12 h to eliminate any residual physisorbed molecules. The specific surface area was determined using the Brunauer–Emmett–Teller (BET) method, while the micropore volume was calculated by applying the Dubinin–Radushkevich (DR) model to the N₂ adsorption data. To assess narrow micropores, the DR equation was also applied to the CO₂ adsorption isotherms.
Morphological features were examined using scanning electron microscopy (SEM) with a LEO GEMINI-1530 system (Carl Zeiss), complemented by optical microscopy observations performed with an OLYMPUS BX51 microscope.
Fourier-transform infrared (FTIR) spectroscopy was conducted using a NICOLET IR200 spectrometer, covering the spectral range of 400–4000 cm⁻^1^. The data were expressed in terms of percentage transmittance.
Surface chemical composition and electronic states were investigated via X-ray photoelectron spectroscopy (XPS), using an ESCA 5701 system (Physical Electronics) equipped with a magnesium Kα source (PHI 04–548) and a hemispherical analyzer. The C 1 s peak at 284.6 eV served as the internal reference for binding energy calibration. Spectral deconvolution was performed using Gaussian-Lorentzian functions and least-squares fitting via the XPSPEAK41 software.
Electrochemical characterization: oxygen electro reduction
To prepare the bio-carbon samples for electrochemical evaluation in oxygen reduction reaction (ORR) studies, 1 mL of Nafion solution was mixed with 9 mL of deionized water and sonicated for 10 min. Subsequently, 5mg of bio-carbon was added to 1 mL of this dispersion, followed by an additional 30-min sonication to ensure uniform suspension. A 20-μL aliquot of the final mixture was carefully dropped-cast onto the disk surface of a rotating ring-disk electrode (RRDE) for ORR testing.
Electrochemical measurements were carried out using an Autolab electrochemical system equipped with a PGSTAT101 potentiostat/galvanostat. Experiments were performed in a conventional three-electrode setup at ambient temperature, employing a platinum sheet as the counter electrode and an Ag/AgCl electrode as the reference. The electrolyte consisted of 100 mL of 0.1 M KOH solution. Prior to each test, the electrolyte was saturated with either nitrogen or oxygen gas by bubbling for 30 min.
Cyclic voltammetry (CV) was conducted over a potential window from −0.80 to 0.40 V at a scan rate of 50 mV/s, with the electrode rotating at 1000 rpm. Linear sweep voltammetry (LSV) was performed under identical potential and scan rate conditions, with rotation speeds ranging from 500 to 3500 rpm to evaluate ORR kinetics.
The number of electrons transferred per oxygen molecule (n) and the hydrogen peroxide selectivity were calculated using established equations (Jiang et al. 2019; Zhou et al. 2016), providing insight into the catalytic efficiency and reaction pathway.
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$n= \frac{4 \times {I}_{\mathrm{D}}}{{I}_{\mathrm{D}}+\frac{{I}_{\mathrm{R}}}{{\mathrm{N}}_{\mathrm{C}}}}$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\%}{\mathrm{H}}_{2}{\mathrm{O}}_{2}= \frac{200\times \frac{\mathrm{I}_{\mathrm{R}}}{\mathrm{N}_{\mathrm{C}}}}{\mathrm{I}_{\mathrm{D}}+\frac{\mathrm{I}_{\mathrm{R}}}{\mathrm{N}_{\mathrm{C}}}}$$\end{document}where ID and IR are the disk and ring currents, respectively, and NC is the collection efficiency of the RRDE (0.249).
The current density (JK) was obtained by Koutecky-Levich equation (Eq. 5).
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\frac{1}{J}= \frac{1}{\mathrm{J}_{\mathrm{K}}}+\frac{1}{\mathrm{B}\times {{{w}}}^{1/2}}$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm{B}=\left(0.62\right)\mathrm{n}\times \mathrm{F}\times \mathrm{A}\times \mathrm{D}^{2/3}\times \mathrm{v}^{-1/6}\times \mathrm{C}$$\end{document}In the applied formulation, Faraday’s constant (F) is defined as 96,485 C/mol, representing the electric charge per mole of electrons. The parameter n corresponds to the number of electrons transferred per oxygen molecule during the oxygen reduction reaction (ORR). A, the geometric surface area of the rotating disk electrode, is 0.2475 cm^2^. D, the diffusion coefficient of molecular oxygen in the alkaline medium, is taken as 1.9 × 10⁻^5^ cm^2^/s, while v, the kinematic viscosity of the electrolyte, is assumed to be 0.01 cm^2^/s. C, the solubility of oxygen in the 0.1 M KOH solution, is considered to be 1.2 × 10⁻⁶ mol/cm^3^, as reported in previous studies (Brocato et al. 2012; Xu et al. 2017).
Electro-Fenton process
The electro-Fenton experiments were performed in a conventional three-electrochemical cell with a total volume of 100 mL, maintained at ambient temperature. Tetracycline (TC) was used as the target pollutant at an initial concentration of approximately 40 mg L⁻^1^, with 0.5 M Na₂SO₄ serving as the supporting electrolyte under continuous magnetic stirring. The electrochemical system was operated in potentiostatic mode at an applied potential of − 0.6 V.
The working electrode was fabricated by blending the doped bio-carbon (DBC) material with polytetrafluoroethylene (PTFE) in a 90:10 mass ratio. The resulting composite was dried at 100 °C for 12 h and subsequently coated onto both sides of a graphite substrate (3 cm × 1 cm), with 25 mg of material applied per face. A platinum sheet was employed as the counter electrode, and an Ag/AgCl electrode served as the reference. All experiments were conducted at the natural pH of the solution.
To monitor TC degradation, 2-mL aliquots were collected at 5-min intervals. The concentration of TC was quantified using UV-Vis spectrophotometry at its maximum absorbance wavelength of 356.5 nm. The degradation efficiency (D%) was calculated according to Eq. (7).
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\%}{Degradation}=\frac{{C}_{0}-{C}_{t}}{{C}_{0}}\times100$$\end{document}where C₀ is the initial concentration of TC (mg L⁻^1^), and Cₜ is the concentration of TC at time t (mg L⁻^1^).
This equation quantifies the reduction in TC concentration over time, providing a measure of the degradation efficiency of the electro-Fenton process.
Results and discussion
Morphological characterization
Figure 1 displays scanning electron microscopy (SEM) micrographs of the pristine bio-carbon (BC) and nitrogen-doped bio-carbon (DBC-N) samples. The DBC-N material was selected for comparative analysis due to its superior electrochemical performance, allowing for a clear assessment of morphological changes induced by nitrogen doping. The SEM images reveal distinct structural differences between the undoped and doped samples. Both materials exhibit irregular surfaces characterized by cracks and pores of various dimensions, indicative of partial degradation within the carbon matrix. However, the DBC-N sample presents a more extensively disrupted surface morphology, which is likely attributable to the incorporation of nitrogen-containing precursors during synthesis. This modification appears to promote the formation of larger voids and interconnected porous networks, contributing to a more pronounced three-dimensional structure.Fig. 1SEM microphotographs (10.00 KX) showing the morphology of samples A BC and B DBC-N
Textural characterization
Figure 2A and Table 1 present the nitrogen adsorption–desorption isotherms for the synthesized materials. According to the IUPAC classification, the pristine bio-carbon (BC) and nitrogen-doped sample (DBC-N) exhibit hybrid type-I/type-IV isotherms, indicative of a hierarchical porous structure comprising both micropores and mesopores (Alcañiz-Monge et al. 2010).Fig. 2. Isotherms N_2_ adsorption (A) and pore size distribution of all samples (B) (blue square, BC; black triangle, DBC-B; red diamond, DBC-N; Ж, DBC-P; green circle, DBC-S)Table 1. Textural characteristics of all the samples obtained by N_2_ adsorption isotherms at 77 K and CO_2_ at 0 °C. Units: S (m^2^ g^−1^); W (cm^3^ g^−1^); L (nm)SampleN_2_CO_2_SB.E.TSD.F.TW0L0V0.95VmesoVD.F.TL0 (D.F.T)W0L_0_BC129714940.490.800.630.140.641.220.440.73DBC-B1751480.071.420.110.040.120.610.190.80DBC-N106111600.421.030.500.080.510.610.400.74DBC-P470.002.050.020.010.022.840.070.71DBC-S114970.051.510.070.020.070.850.140.90
The boron- and sulfur-doped samples (DBC-B and DBC-S) display type-IV isotherms, characteristic of mesoporous frameworks, whereas the phosphorus-doped sample (DBC-P) shows a type-III isotherm, typically associated with nonporous or weakly adsorbing surfaces. A general decrease in nitrogen uptake at low relative pressures was observed across all doped samples, suggesting that heteroatom incorporation partially obstructs the pore network, thereby reducing accessible surface area. Among the dopants, nitrogen appears to preserve the porous architecture most effectively, introducing functional groups while maintaining structural integrity. In contrast, other dopants tend to compromise porosity through pore collapse or blockage.
Carbon dioxide adsorption was employed to probe ultra-microporosity, targeting pores narrower than 0.7 nm. In contrast, nitrogen adsorption provides insight into super-microporosity under unrestricted diffusion conditions. For all doped samples, the micropore volume derived from N₂ adsorption (W₀(N₂)) was lower than that obtained from CO₂ adsorption (W₀(CO₂)), indicating the presence of narrow pores or restricted pore entrances inaccessible to N₂ molecules. Additionally, the average pore width (L₀(N₂)) increased, consistent with partial obstruction of finer pores. These findings suggest that surface modification reduces both surface area and total pore volume, while increasing the mean pore diameter.
Figure 2B illustrates the micropore size distribution for all samples. The modified materials exhibit a narrow distribution centered around 0.6 nm. The intensity of this peak is highest for BC and DBC-N and significantly diminished in the other doped samples. This reduction implies that surface modification may have led to the destruction or merging of adjacent micropores, resulting in a loss of micropore volume and surface area due to structural degradation within the carbon matrix.
Chemical characterization
Figure 3 displays the Fourier transform infrared (FTIR) spectra of the synthesized materials, offering insight into the surface functional groups introduced through heteroatom doping. All samples exhibit characteristic vibrational bands corresponding to C–O stretching (900–1300 cm⁻^1^), C=C skeletal vibrations (1500–1600 cm⁻^1^), and O–H stretching (around 3500 cm⁻^1^), consistent with previously reported data (Kalaiyarasan et al. 2020; Li et al. 2013; Mojoudi et al. 2019; Saka 2012). Additionally, absorption bands associated with CO and CO₂ were observed near 2100 cm⁻^1^ and 2400 cm⁻^1^, respectively (Wang et al. 2023).Fig. 3FTIR of (blue square) BC, (black triangle) DBC-B, (red diamond) DBC-N, (Ж) DBC-P, and (green circle) DBC-S
Further analysis of the doped samples reveals distinct spectral features attributable to the presence of heteroatoms. In the DBC-N sample, a broad absorption band near 3450 cm⁻^1^ is assigned to overlapping –O–H and –N–H stretching vibrations. A weak band at 2920 cm⁻^1^ corresponds to C–H stretching, while a prominent peak near 1600 cm⁻^1^ reflects C-H bending. The broad signal at 1100 cm⁻^1^ is indicative of C–N stretching, and the peak at 1630 cm⁻^1^ is associated with aromatic C=C and C=N vibrations. Additional nitrogen-related features include N–H stretching at 3220 cm⁻^1^, in-plane N–H deformation at 1530 cm⁻^1^, and C–N stretching at 1455 cm⁻^1^. A minor peak at 880 cm⁻^1^, attributed to N–C bonding, likely arises from C–O–C linkages. The sharp band at 1502 cm⁻^1^ suggests the presence of nitrogen-containing groups such as C–N and N–O.
In the DBC-S sample, sulfur incorporation leads to enhanced water-related absorption in the 1650–1885 cm⁻^1^ region, likely due to increased polarity within the carbon matrix. Characteristic sulfur vibrations include SO₂ symmetric stretching (1120–1190 cm⁻^1^), S = O stretching (1020–1060 cm⁻^1^), C–S stretching (600–700 cm⁻^1^), and S–S stretching (450–550 cm⁻^1^).
The FTIR spectrum of DBC-B reveals distinct bands at 1120, 1168, 1397, and 696 cm⁻^1^, corresponding to B–C stretching, B–O–H bending, B–O stretching, and O–B–O vibrations, respectively, confirming successful boron incorporation into the carbon framework.
For the DBC-P sample, a notable band at 1438 cm⁻^1^ is attributed to asymmetric C–C stretching within phenyl rings, which may also indicate the presence of phenyl–phosphorus groups. A band at 803 cm⁻^1^ corresponds to C–P bonding. The increased intensity of the broad band at 3423 cm⁻^1^, relative to the BC sample, suggests a higher concentration of hydroxyl groups.
The FTIR spectra confirm the successful functionalization of the bio-carbon matrix with heteroatoms, each imparting distinct chemical signatures and contributing to the structural and catalytic diversity of the materials.
The surface chemical composition and electronic states of the elements present in the samples were investigated by X-ray photoelectron spectroscopy (XPS), focusing on the core-level spectra of C_1s_ and O_1s_, as shown in Fig. 4. The binding energy (BE) scale was calibrated using the graphitic carbon C_1s_ peak at 284.6 eV as an internal reference. Deconvolution of the C_1s_ region revealed multiple components corresponding to distinct bonding environments: C = C (284.6 eV), C–C (285.6 eV), C–O (286.9 eV), C = O (287.7 eV), COO⁻ (289.4 eV), and π–π* transitions (290.9 eV), consistent with previous reports (Elmouwahidi et al. 2018a, b).Fig. 4C_1s_ and O_1s_ XPS spectra of A) DBC-B, B) DBC-N, C) DBC-P, and D) DBC-S
The O_1s_ spectra exhibited peaks attributable to oxygen-containing functional groups, primarily C = O and C–OH, located at approximately 531.0 eV and 533.0 eV, respectively (Table 2 and Fig. 4). In the case of the boron-doped sample (DBC-B), the presence of B–O bonds contributed to a more intense signal near 532.9 eV, reflecting the higher boron content relative to other doped samples. For the nitrogen-doped sample (DBC-N), the O–N component was identified at 533.1 eV, slightly shifted compared to DBC-B, indicating the incorporation of nitrogen into the oxygen coordination environment. Table 2C1s and O1s XPS summary of all samplesElementPeakBC (% peak)DBC-B (% peak)DBC-N (% peak)DBC-P (% peak)DBC-S (% peak)C1sC=C6660567664C-C1721161314C-O76846C=O56625COO-44844π-π*14554O1sC=O3943524953C-OH6157485147
In the phosphorus-doped sample (DBC-P), the P = O bond was detected within the peak at 531.2 eV, while additional phosphorus-related functionalities such as C–O–P–O and C–PO were observed within the broader signal at 533.0 eV, in agreement with previous works (S.K.Das et al. 2012). These spectral features confirm the successful incorporation of heteroatoms into the carbon matrix and provide insight into the chemical environments introduced through doping (Figs. 5 and 6).Fig. 5XPS spectrum of each respective heteroatom in the samples A) DBC-B, B) DBC-N, C) DBC-P, and D) DBC-SFig. 6CV of A BC, B DBC-B, C DBC-N, D DBC-P, and E DBC-S; O_2_ (red) and N_2_ (blue)
The analysis of the spectrums was summarized in Tables 2 and 3, which show the different percentages of each component and element analyzed by XPS. Table 3XPS summary of the different heteroatoms and percentage of heteroatom in all sampleSamplePeak% PeakElement Wt.%DBC-B190.8225.6192.464193.915DBC-N398.9474.1400.140401.213DBC-P132.1554.3133361349DBC-S163.5252.2164.73168.127169.215
The results indicate that the DBC-N and DBC-B samples exhibit a higher concentration of carbonyl groups (C = O) at 287.4 eV in the C1s spectrum and 531.1 eV in the O1s spectrum, respectively (Velo-Gala et al. 2014), which are known to enhance oxygen reduction reaction (ORR) activity (W. Zhou et al. 2021). In the DBC-B sample, the presence of B-CO₂ (193.9 eV), BC₂O (192.4 eV) and BC3 (190.8 eV) bonds suggests the incorporation of boron into the carbon matrix (Matsoso et al. 2022; Zheng et al. 2022). The DBC-P sample displays C-P (132.1 eV), C-PO_3_ (133 eV), and C-O-PO₃ (134 eV) bonds, indicating the presence of phosphorus-containing functional groups (Elmouwahidi et al. 2017; Kalaiyarasan et al. 2020). In the DBC-S sample, the detection of C-SH (163.5 eV), R₂-SO (164.7 eV), C-OSO₃H (168.11 eV), and C-O-SO₂-O-C (169.23 eV) bond points to the incorporation of sulfur-containing groups (Elmouwahidi et al. 2016; Gooch and Hlady 2015). The N1s spectrum of DBC-N reveals peaks corresponding to N-pyridine (398.9 eV), N-pyrrole (399.6 eV), and N-graphitic (400.9 eV) nitrogen species (Cao et al. 2018; Y. Li et al. 2017). Studies have shown that pyridinic nitrogen sites are associated with selectivity toward hydrogen peroxide (H₂O₂) production, while pyrrolic nitrogen sites are more active for water (H₂O) production during the ORR (Z. Xing et al. 2021).
It is important to highlight that a relatively high number of heteroatoms (around 4–5 wt. %) are fixed on the carbon matrix, especially for B, N, and P samples doped. However, this high doping is to the detriment of the porosity for all samples except for the N-doping sample where a high number of N-groups are fixed on the surface still preserving a well-developed porosity of Bio-carbon.
Electrochemical characterization
To investigate the catalytic behavior of the synthesized materials toward the oxygen reduction reaction (ORR), linear sweep voltammetry (LSV) measurements were performed at varying electrode rotation rates, as illustrated in Fig. 7. The resulting current–potential profiles were analyzed using the Koutecky–Levich (K–L) equation to determine the number of electrons transferred per oxygen molecule at different applied potentials. The calculated electron transfer numbers are presented in Fig. 8.Fig. 7LSV of A BC, B DBC-B, C DBC-N, D DBC-P, and E DBC-S; at different RPM 1000, 2000, 3000, and 3500Fig. 8A LSV curves to 3500 rpm, B number of electrons transferred, and C selectivity to H_2_O_2_ of (blue square) BC, (black triangle) DBC-B, (red diamond) DBC-N, (*) DBC-P, and (green circle) DBC-S
This analysis provides valuable insight into the ORR kinetics and mechanistic pathways associated with each doped bio-carbon sample. The variation in electron transfer values across the potential range reflects differences in catalytic efficiency and selectivity, thereby enabling a comparative assessment of the materials’ suitability for specific electrochemical applications, such as energy conversion and environmental remediation.
The catalytic mechanism is crucial for determining the applications of the samples, as the oxygen reduction reaction (ORR) can proceed via two pathways: (Eqs. 8 and 9) (Bajracharya et al. 2016; Qin & Zhao 2022), the two-electron pathway is particularly relevant for electro-Fenton processes.
To elucidate the catalytic mechanism of the samples, linear sweep voltammetry (LSV) experiments were conducted at varying rotation speeds, as shown in Fig. 7. The Koutecky-Levich equation was employed to analyze the resulting curves and calculate the number of electrons transferred across different potentials for all samples, as depicted in Fig. 8. This analysis provides insights into the electron transfer processes occurring during the ORR and aids in assessing the suitability of the samples for specific applications.
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{O}}_{2}+{4\mathrm{H}}^{+}+{4\mathrm{e}}^{-}\to {2\mathrm{H}}_{2}\mathrm{O}$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{O}}_{2}+{2\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to {\mathrm{H}}_{2}{\mathrm{O}}_{2}$$\end{document}Figure 8 presents the linear sweep voltammetry (LSV) curves recorded at a rotation speed of 3500 rpm, alongside the calculated number of electrons transferred and the selectivity toward hydrogen peroxide (H₂O₂) for each sample. Among the tested materials, the nitrogen-doped bio-carbon (DBC-N) exhibited the highest current density, highlighting its potential for cost-effective and scalable applications in electrocatalysis (Zhang et al. 2022a, b, c). The electron transfer number for all samples ranged between 2.65 and 2.80, indicating a predominant two-electron oxygen reduction pathway conducive to H₂O₂ generation. As shown in Table 4 and Fig. 8A, heteroatom doping significantly enhances the selectivity toward the two-electron ORR mechanism. Notably, nitrogen doping particularly in the form of pyrrolic nitrogen has been reported to lower the activation energy barrier, thereby facilitating a three-electron pathway and improving overall catalytic activity. Similarly, phosphorus doping introduces electron-donating effects that alter the carbon hybridization from sp^2^ to sp^3^, creating additional active sites favorable for ORR (Huang et al. 2023; Lu et al. 2021; Xia et al. 2021). Table 4. Summary of electrochemical parameters derived from LSV analysis: onset potential (Eₒₙₛₑₜ), electron transfer number (n), and kinetic current density (Jₖ)SampleJ_K_ (mA cm^**−2^)*nE*°onset (V)BC5.572.65 − 0.15DBC-B9.562.76 − 0.15DBC-N10.382.79 − 0.14DBC-P8.612.69 − 0.20DBC-S9.382.80 − 0.19
These findings underscore the critical role of heteroatom doping in modulating the electronic structure and surface chemistry of carbon-based materials, thereby enhancing their electrocatalytic performance in oxygen reduction reactions.
Figure 9 displays the Tafel plots derived from the specific kinetic current densities of the synthesized samples, which are essential in determining the Tafel slope, a key parameter for evaluating the kinetics of the oxygen reduction reaction (ORR). This approach enables comparative analysis of the electrocatalysts based on their oxygen reduction rates, thereby facilitating the identification of materials with superior catalytic performance (A. Das et al. 2023; Kiran et al. 2023).Fig. 9. Tafel plot of specific kinetic current density for (blue square) BC, (black triangle) DBC-B, (red diamond) DBC-N, (*) DBC-P, and (green circle) DBC-S
In electrochemical systems, the Tafel slope reflects the rate at which current density increases with overpotential. Lower slope values are indicative of faster reaction kinetics and enhanced electrocatalytic activity. A slope of 120 mV dec⁻^1^ typically corresponds to a rate-determining first electron transfer step. A value of 60 mV dec⁻^1^ suggests that the initial electron transfer is followed by a chemical step, while a slope of 40 mV dec⁻^1^ implies that the second electron transfer governs the reaction rate.
The Tafel slopes obtained for the samples were as follows: 45.21 mV dec⁻^1^ for BC, 44.33 mV dec⁻^1^ for DBC-B, 30.35 mV dec⁻^1^ for DBC-N, 23.50 mV dec⁻^1^ for DBC-P, and 26.34 mV dec⁻^1^ for DBC-S. These results demonstrate that the phosphorus-doped sample (DBC-P) exhibits the most favorable ORR kinetics among the tested materials. Overall, the Tafel slope serves as a reliable indicator of electrocatalytic efficiency and mechanistic insight into the ORR pathway.
Electro-Fenton process
All samples were selected for evaluation of TC degradation via the EF process. The bio-carbons were assessed without a Fenton catalyst to determine potential degradation in a single step using a unique catalyst capable of generating H₂O₂ and activating it to produce hydroxyl radicals (•OH) (Fig. 10).Fig. 10. Degradation of tetracycline by electro-Fenton with (dark orange circle) graphite (support), (blue square) BC, (black triangle) DBC-B, (red diamond) DBC-N, (*) DBC-P, and (green circle) DBC-S
Figure 10 illustrates the degradation efficiency of tetracycline (TC) across all samples, with the highest removal rates observed for the sulfur- and nitrogen-doped bio-carbon catalysts (DBC-S and DBC-N). Based on the combined results from the oxygen reduction reaction (ORR) and electro-Fenton experiments, DBC-N emerges as the most effective catalyst, as evidenced by its superior kinetic current density (Jₖ) reported in Table 4 and its enhanced TC degradation performance.
The improved activity of DBC-N is attributed to the presence of graphitic-N and pyridinic-N functionalities, which play distinct roles in the electrochemical process. Graphitic-N sites are known to promote the two-electron ORR pathway, facilitating H₂O₂ generation, while both graphitic-N and pyridinic-N contribute to the activation of H₂O₂ into hydroxyl radicals (•OH), a key step in the electro-Fenton mechanism (Su et al. 2019). Additionally, graphitic-N and nitrogen vacancies have been proposed as active sites for O₂ adsorption and (•O₂⁻) formation, further enhancing catalytic performance.
Sulfur doping also contributes positively to ORR activity by increasing the adsorption energy of O₂ molecules, thereby promoting the two-electron reduction pathway (Zhu et al. 2021). However, sulfur does not significantly influence the desorption of the OOH intermediate, a critical step in H₂O₂ selectivity. These findings suggest that both nitrogen and sulfur doping strategies are effective in enhancing ORR activity and electro-Fenton degradation, with comparable performance metrics observed for DBC-N and DBC-S in terms of selectivity and TC removal efficiency.
The degradation mechanism proposed in this study, based on scientific literature, is mainly mediated by the initial electrophilic attack of hydroxyl radicals on the sites of highest electron density in the TC molecule, such as the C3 double bond and the phenolic groups (Fang et al. 2022). Subsequently, demethylation reactions in the amino group and hydroxylation processes of the aromatic rings have been reported (Dong et al. 2025). These stages lead to the fragmentation of the multicyclic structure of TC, generating lower molecular weight intermediates and short-chain organic acids prior to their subsequent mineralization (Li et al. 2022). Although additional mass spectrometry analyses are required to confirm the specific intermediate products, this proposed pathway is consistent with the reduction in TC concentration measured by UV-Vis.
To contextualize the efficiency of the doped bio-carbons obtained from alperujo, the results were compared with recent studies on heterogeneous electro-Fenton processes. For instance, the use of nickel foams modified with nitrogen-doped carbon structures (CoFe@NC) has been reported to achieve efficient degradation of organic pollutants over a wide pH range, highlighting the importance of nitrogen active sites (Sun et al. 2022). Similarly, the use of cathodes based on layered double hydroxides (CoFe-LDH) has demonstrated that catalyst architecture is crucial for electron transfer and radical generation (Yu et al. 2021). In comparison, our DBC-N and DBC-S materials not only achieve competitive degradation capacities (up to 70% for TC) but do so using agro-industrial waste as a single precursor. This represents a significant advantage in terms of sustainability and circular economy compared to systems relying on transition metals encapsulated in carbon aerogels (Xiao et al. 2021). This metal-free bifunctional nature simplifies the process design and reduces the risk of metal leaching into the aquatic environment.
Conclusions
This study successfully demonstrates the synthesis and heteroatom functionalization of bio-carbon (BC) derived from agro-industrial waste, employing nitrogen (N), boron (B), phosphorus (P), and sulfur (S) as dopants. X-ray photoelectron spectroscopy (XPS) confirmed effective doping across all samples, with incorporation levels near 5%, except for DBC-S, which contained approximately 2% sulfur. These results validate the reliability of the doping strategy used.
Comprehensive physicochemical characterization—including N₂ and CO₂ adsorption isotherms, SEM, FTIR, and XPS—revealed that nitrogen doping best preserved the porous architecture, while other dopants introduced varying degrees of pore blockage and surface modification. Functional group analysis confirmed the presence of dopant-specific chemical environments, contributing to the catalytic behavior of the materials.
Electrochemical evaluation via cyclic voltammetry (CV), linear sweep voltammetry (LSV), and rotating ring-disk electrode (RRDE) techniques demonstrated enhanced oxygen reduction reaction (ORR) activity for all doped samples. DBC-N exhibited the highest kinetic current density (Jₖ) and H₂O₂ selectivity, attributed to the presence of graphitic-N and pyridinic-N sites, which facilitate both H₂O₂ generation and activation into hydroxyl radicals (•OH). Sulfur doping also showed promising ORR performance, likely due to improved O₂ adsorption energy, supporting a two-electron reduction pathway.
The electrocatalytic efficiency of the materials was further validated through electro-Fenton degradation of tetracycline (TC), with DBC-N and DBC-S achieving nearly 70% degradation within 300 min. This performance is attributed to the generation of reactive oxygen species, particularly •OH, via a three-electron ORR pathway.
Overall, the findings highlight the potential of heteroatom-doped bio-carbons as sustainable, metal-free catalysts for environmental remediation and energy conversion applications. The integration of low-cost synthesis with tailored surface chemistry offers a promising route for the development of advanced functional materials with high relevance in catalysis, energy transformation, and organic pollutant degradation.
The reference list from the paper itself. Each links out to its DOI / PubMed record.
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