Synergistic impact of rice husk biomass derived carbon supports on the performance of biogenic Fe⁰-catalyzed advanced oxidation processes for oxytetracycline remediation

Abstract

This study explores the use of rice husk biomass and its derived carbon materials—hydrochar (HC) and biochar (BC)—as supports for biogenic zerovalent iron (ZVI) nanocomposites (ZVI@RH, ZVI@HC, and ZVI@BC) in advanced oxidation processes (AOPs) for the degradation of oxytetracycline (OTC). The catalysts were characterized using FTIR, XRD, FESEM, and XPS techniques, and their performance in activating peroxymonosulfate (PMS) for OTC degradation was assessed. Results showed that the ZVI@BC nanocomposite outperformed ZVI@RH and ZVI@HC in OTC removal through heterogeneous Fenton-like processes. The addition of PMS further enhanced OTC degradation by generating more reactive oxygen species (ROS), making the process more efficient than the Fenton process alone. The higher surface defects in BC, resulting from pyrolysis, improved OTC adsorption and degradation, and facilitated more effective ZVI-mediated PMS activation in ZVI@BC, achieving nearly 98.3% OTC removal from the aqueous solution. The involvement of various ROS in OTC degradation was examined using radical scavengers, and DFT calculations proposed a degradation pathway by identifying ROS attack sites on the OTC chromophore. High-resolution mass spectrometry (HRMS) analysis was used to identify reaction intermediates. This study emphasizes the potential of using agricultural waste-derived materials in AOPs, presenting a sustainable and cost-effective method for environmental remediation and OTC antibiotic degradation.

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Water impact

This study highlights the environmental significance of utilizing rice husk biomass and its derived carbon materials—hydrochar (HC) and biochar (BC)—as effective supports for biogenic zerovalent iron (ZVI) nanocomposites in advanced oxidation processes (AOPs). By utilizing agricultural waste, this research not only contributes to waste valorization but also provides a sustainable approach to addressing water contamination, particularly from pharmaceuticals like oxytetracycline (OTC). The findings demonstrate that the ZVI@BC nanocomposite significantly enhances the degradation of OTC through the generation of reactive oxygen species (ROS) when activated by peroxymonosulfate (PMS). This innovative method achieves nearly 98.3% removal of OTC from aqueous solutions, showcasing its potential for practical applications in environmental remediation. The study's focus on the mechanistic pathways of OTC degradation furthers our understanding of AOPs and supports the development of cost-effective and efficient strategies for mitigating antibiotic pollution in water bodies. By integrating the principles of green chemistry and waste reuse, this research not only addresses a pressing environmental challenge but also promotes the sustainable use of natural resources, contributing to the broader goals of pollution reduction and ecosystem health.

1. Introduction

The contamination of pharmaceutical residues in water bodies poses a significant challenge for the environment and human health. Oxytetracycline (OTC), a widely used broad-spectrum antibiotic in veterinary medicine, is particularly concerning due to its persistence and potential ecological impact.¹ Oxytetracycline is a widely used antibiotic in pig farming, and has been detected in various environmental matrices, including groundwater, surface water, soils, and sediments.² Oxytetracycline is one of the second most broadly used classes of TCs due to its antimicrobial effects and lower cost.³ The low oral bioavailability (30–58%) of OTC antibiotics leads to high residual concentrations in farm wastewater and they are detected in water bodies worldwide due to their long history of use.⁴ Oxytetracycline, the most persistent member of antibiotics, is capable of remaining stable in subsurface soils for several months. Its widespread usage, coupled with its detection in effluent from wastewater treatment plants (564.30 ng L⁻¹)⁵ and environmental water samples (388 μg L⁻¹),⁶ raises significant concerns about its environmental and health impacts. OTC's persistence in aqueous environments allows it to remain in water bodies for extended periods, undergoing complex chemical reactions. These properties, along with its potential risks to human and animal health, underscore the urgent need to develop effective treatment strategies for removing OTC from wastewater and industrial effluents, thereby mitigating its environmental footprint.⁷

In recent years, various techniques including advanced oxidation processes,^8,9 photocatalytic degradation, biodegradation, and adsorption^10,11 have frequently been used to provide scientific solutions for OTC contamination. Among these methods, adsorption is considered a favourable approach for removing antibiotics due to its efficiency, cost-effectiveness, eco-friendliness, scalability, and lower toxicity.⁸ Furthermore, advanced oxidative processes (AOPs) are superior to adsorption because they not only remove but also destroy contaminants through chemical reactions, transforming them into harmless by-products.¹² However, the combination of adsorption and AOPs enhances overall treatment efficiency, reduces operational costs, and provides a versatile solution for various environmental remediation challenges.

Agricultural wastes can be utilized as low-cost sorbents for environmental remediation. These agricultural biomass waste materials are abundant, renewable, and low cost, and have a high surface area and porous structure, and contain various functional groups. They can adsorb a variety of pollutants, including heavy metals, dyes, and organic compounds from water. The utilization of agricultural wastes for sorption purposes adds value to the reduction of waste disposal issues, and promotes sustainable waste management practices. Further, the modification and activation of biomass wastes can enhance their adsorption capabilities, making them even more efficient for environmental cleanup. In recent years, sulfate radical-based advanced oxidation processes (AOPs) have become increasingly popular due to their reactivity and longevity.⁸ Advanced oxidation processes (AOPs) are one of the most effective methods for the degradation of antibiotics. This study focuses on advanced oxidative processes (AOPs) in which biomass-derived materials are used to activate peroxymonosulfate (PMS). The activation of PMS by the biomass generates reactive radicals capable of degrading a wide range of pollutants into harmless by-products. This approach not only utilizes low-cost and abundant biomass resources but also enhances their functionality beyond mere adsorption to actively degrade pollutants, as in the literature, biomass and its derived materials are used as adsorbents for various pollutants. This addresses the limitations of traditional biomass-derived adsorbents. Further, the immobilization of zerovalent iron (ZVI) onto the matrices of biomass and its derived materials enhances their adsorption capacity and introduces reactive sites for pollutant degradation through Fenton-like reactions. In AOPs, activation of peroxymonosulfate (PMS) is a key step to achieve excellent oxidation performance. Zerovalent transition metal-based nanomaterials are effective for the generation of reactive oxygen species;⁹ their paramagnetic nature often results in reduced surface area and catalytic activity.¹² The immobilization of metals onto organic or inorganic supports enhances their surface activities. Agricultural biowaste-derived products are promising support materials because of their low cost, abundance, biodegradability, and high stability. They offer a sustainable solution for improving the performance of zerovalent metal catalysts in advanced oxidation processes.⁸ Previous studies have used agricultural biowaste-based biodegradable materials as supports for environmental remediation. The biomass-derived material enhanced the contaminant removal process by providing synergistic support to zerovalent iron nanoparticles.⁹ However, there is potential to improve the surface properties of agricultural biowaste materials through physical or chemical pretreatments before using them as support materials in nanocomposite synthesis and to compare the efficiency of ZVI supports with biowaste-derived products.

Rice husk, a by-product of rice milling, is rich in organic and inorganic constituents, making it suitable for environmental remediation. It contains cellulose, hemicellulose, lignin, and 15–20% silica, providing durability for effective remediation. Further, with pyrolysis and hydrothermal carbonization, rice husk can be converted into biochar and hydrochar, respectively. The porous structure and high surface area of rice husk-derived biochar and hydrochar enable efficient adsorption of pollutants. Previous studies by the authors have utilized biodegradable materials derived from agricultural biowaste to support the synthesis of immobilized metal or metal oxide nanoparticles. These nanoparticles have been employed for the remediation of various contaminants, including inorganic hexavalent chromium,¹⁰ organic dyes like crystal violet and rhodamine B,⁹ and active pharmaceuticals such as terbinafine¹³ and tetracycline.¹⁴ This study utilized rice husk, hydrochar, and biochar with enhanced surface adsorption properties to immobilize zerovalent iron nanoparticles. These nanoparticles were produced using a sustainable method involving the reduction process with Melia azedarach (Linn.) also commonly known as chinaberry, under hydrothermal conditions forming ZVI@RH, ZVI@HC, and ZVI@BC nanocomposites. The need for effective environmental remediation technologies has grown with increasing pollution levels and environmental degradation. This study investigates the effectiveness of a new system utilizing biomass and its derived carbonaceous materials for immobilizing ZVI to give nanocomposites that are used for removing OTC from water through adsorption and degradation processes. By exploring their application prospects and understanding the underlying mechanisms of pollutant degradation, we aim to offer insights into how these materials can contribute to more efficient and sustainable solutions for environmental protection. The metal immobilized in carbon materials can efficiently act as electron transfer sites, and thus significantly improve the efficiency of nanocomposite catalysts. The high OTC adsorption efficiencies of rice husk and its carbonaceous materials complement the oxidative degradation tendencies of ZVI@RH, ZVI@HC, and ZVI@BC mediated heterogeneous Fenton processes, and the ability of zerovalent iron to activate PMS to produce active reactive oxygen species (ROS) leads to a synergistic effect that enhances the overall degradation of OTC in wastewater. By immobilizing ZVI within the biomass and its derived carbonaceous material matrix, the system aims to maximize the utilization of both materials while ensuring their stability and reusability.

2. Reagents and materials

2.1. Preparation of rice husk powder (RH)

Rice husk, a by-product of rice processing, was obtained from a local rice husk mill. It was rinsed several times with distilled water to wash the dust and impurities from the surface and was subjected to air dry in an oven at 100 °C for 24 h. The dried material was ground and sieved to obtain a homogeneous powder labelled as rice husk powder (RH).

2.2. Rice husk to biochar (BC) and hydrochar (HC) conversion

Rice husk biochar (BC) was obtained by carbonization of rice husk powder (RH) at 600 °C for 3 h under N₂ flow (50 mL min⁻¹) using a tubular furnace. The black residue was washed with ample distilled water and air dried in an oven for 24 h at 100 °C.

Rice husk hydrochar was obtained by subjecting the rice husk powder to hydrothermal treatment (RH : H₂O, 1 : 3), in a steel autoclave at 180 °C for 6 h.

2.3. Preparation of Melia azedarach (Linn.) leave extract

Melia azedarach (Linn.) leaves were obtained from a chinaberry tree grown in the local field, and washed with distilled water. The leaves were shade-dried and ground to make a powder. 50 g of leaf powder was refluxed with 150 mL of distilled water for 2 h. The resulting extract was then filtered and stored at 5 °C for future use.

2.4. Biogenic synthesis of ZVI@RH, ZVI@HC, and ZVI@BC composites

The green synthesis approach was used to prepare zerovalent iron immobilized nanocomposites of rice husk (ZVI@RH), hydrochar (ZVI@HC), and biochar (ZVI@BC). In a typical synthesis method, 5 g of RH was mixed with 25 mL aqueous leaf extract of Melia azedarach (Linn), 10 mL solution of FeCl₃·H₂O (1.93 mM), and 1 mL of hydrazine. The mixture was given hydrothermal treatment at 120 °C for 3 h in an PTFE container enclosed in a steel autoclave. The black mass was filtered and washed with MeOH–H₂O (1 : 1) to remove the residual leaf extract. The green synthesized ZVI@RH was vacuum-dried, and stored in an airtight container till further use. Similar experimental conditions were used to synthesize ZVI@HC and ZVI@BC nanocomposites using hydrochar (HC) and biochar (BC).

2.5. Physical characterization of ZVI@RH, ZVI@HC, and ZVI@BC

Fourier transform infrared (FTIR) spectroscopy of lignocellulose biomass (RH) and its derived carbon materials (HC and BC) and its nanocomposites immobilized with Fe⁰ was used to determine the characteristic bonds. The crystal structure and phase of nanocomposites were determined using the powder X-ray diffraction (XRD) technique at 2θ angles ranging from 10 to 80°. A field emission scanning electron microscope with an integrated energy dispersive X-ray (EDAX) analyser was used to determine the surface morphology of nanocomposites and ImageJ software was used to analyse FESEM data. The high-resolution mass spectrometry of the samples was performed using a QTOF mass spectrometer with a UPLC combined ESI and APCI sources with +ve and −ve mode scans. Atomic absorption spectrometry (AAS) is used to determine the concentration of metal elements in a sample. The elemental composition and charge on the valence state of zerovalent metal immobilized on the surface of biomass were analysed using the X-ray photoelectron spectroscopy (XPS) technique. A UV-vis spectrophotometer was used to determine the concentration of the solution. The equipment utilized for the experiment includes an incubated shaker, centrifuge, hot air oven, pH meter, and vacuum pump (Table S1†).

3. Results and discussion

3.1. Fourier transform infrared (FTIR) analysis

The Fourier-transform infrared (FTIR) spectroscopic analysis of rice husk powder (RH) biomass typically shows specific absorption bands corresponding to functional groups such as hydroxyl (–OH), methyl (C–H), carbonyl (C=O), carboxyl (COOH), aromatic C=C, aromatic C–H, and silicone hydroxide and oxide (Si–OH and Si–O–Si), which are characteristic of cellulose, hemicellulose, lignin and silica constituents (Fig. 1).¹⁵ A broad and strong absorption band of RH with a peak at 3416 cm⁻¹ indicates the presence of hydrogen-bonded hydroxyl (–OH) groups on the surface in cellulose, hemicellulose, and lignin, and is attributed to adsorbed water, along with the vibrations of the silanol groups (Si–OH).¹⁶ The peaks observed at 2850 and 2926 cm⁻¹ exhibit C–H symmetric and asymmetric stretching vibrations of aromatic methoxy, methyl, and methylene groups of the side chain of lignin, cellulose, and hemicellulose.¹⁶ The band of RH at 1630 cm⁻¹ was observed for both aliphatic C=C bonds in the organic component and deformation vibrations of water molecules.¹⁷ The band of RH at 1453 cm⁻¹ represents the C=C stretching of the aromatic rings of lignin. A broad absorption band with a peak at 1054 cm⁻¹ is ascribed to the vibrations of the C–OH bond, and the C–O–C skeleton in lignin, cellulose, and hemicellulose, as well as the Si–O–Si bond in the siloxane groups.¹⁸ The peak at 877 cm⁻¹ is assigned to the aromatic C–H bending of lignin.¹⁹ The hydrothermal activation and pyrolysis of rice husk biomass produce hydrochar (HC) and biochar (BC), respectively, and result in the modification of the IR spectrum indicating structural transformations. The FTIR spectrum of HC and BC indicates that thermal activation leads to dehydration, resulting in a decrease in the intensity of the hydroxyl (–OH) peak.²⁰

Fig. 1 FTIR analysis of rice husk powder (RH), rice husk hydrochar (HC), rice husk biochar (BC), zerovalent iron immobilized on rice husk (ZVI@RH), rice husk hydrochar (ZVI@HC) and rice husk biochar (ZVI@BC).

3.2. XRD analysis

The crystalline structures of rice husk biomass (RH), hydrochar (HC), biochar (BC), and their respective zerovalent iron (ZVI) immobilized nanocomposites (ZVI@RH, ZVI@HC, ZVI@BC) were analysed using X-ray diffraction (XRD) pattern analysis (Fig. 2). Biomass and hydrochar exhibited diffuse peaks indicating low crystallinity, primarily from cellulose, appearing around 2θ = 15° and 22°, corresponding to the (002) reflection plane of the crystalline lignocellulosic framework of rice husk powder with JCPDS card no. 03-0289.¹⁰ Upon pyrolysis of rice husk biomass to form biochar, the cellulose peaks diminished, resulting in a broad peak around 2θ = 20–30°, indicative of amorphous carbon.²¹

Fig. 2 XRD analysis of rice husk powder (RH), rice husk hydrochar (HC), rice husk biochar (BC), zerovalent iron immobilized on rice husk (ZVI@RH), rice husk hydrochar (ZVI@HC) and rice husk biochar (ZVI@BC).

The immobilization of ZVI on RH, HC, and BC resulted in the formation of ZVI@RH, ZVI@HC, and ZVI@BC nanocomposites, which exhibited an observed peak at 44.7° characteristic of zerovalent iron indexed to the (110) plane, with lattice parameters and cell volume values consistent with those listed in Table 1. These values match the standard parameters for zero-valent iron (JCPDS card no. 06-0696), specifically with lattice parameters (a = b = c = 2.969 Å) and cell volume (v = 26.17 ų).²² The average crystallite size and interplanar distances for ZVI@RH, ZVI@HC, and ZVI@BC were determined using the Debye–Scherrer equation and Bragg's law, as summarized in Table 1, demonstrating close agreement with standard values for lattice parameters and cell volume.

Table 1 Cell lattice parameters and average crystallite size from XRD data

Sample	d 110 (Å)	Lattice parameters (Å) a = b = c	Unit cell volume (Å^3)	Average crystallite size (nm)
ZVI@RH	2.10	2.97	26.17	3.49
ZVI@HC	2.10	2.97	26.17	3.97
ZVI@BC	2.10	2.97	26.17	4.73

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3.3. FESEM and EDAX analysis

Field scanning electron microscopy (FESEM) and energy dispersive X-ray (EDAX) analyses were employed to examine the morphology and elemental composition of rice husk biomass (RH), hydrochar (HC), biochar (BC), and their corresponding zerovalent iron (ZVI) nanocomposites (ZVI@RH, ZVI@HC, and ZVI@BC) (Fig. 4a–c). Biomass (RH) exhibited a fibrous structure with low porosity, attributed to cellulose, hemicellulose, and lignin. Hydrochar (HC), produced via hydrothermal carbonization, displayed a compact, spherical morphology with increased porosity compared to biomass. Biochar, derived from pyrolysis, showed a porous, honeycomb-like structure indicative of significant carbonization and organic volatilization (Fig. 3a–c). The incorporation of ZVI resulted in dispersed iron nanoparticles within the biomass/char matrix, showing that the existing pores were filled with iron particles attached to the surface of the support. EDAX analysis confirmed these findings by revealing elemental compositions: biomass exhibited peaks for C and O, whereas hydrochar and biochar showed higher carbon content and reduced oxygen signals due to carbonization. In ZVI nanocomposites, prominent Fe peaks in the EDAX spectra confirmed successful iron incorporation onto the surface of biomass and its derivatives (Fig. 4d–f).

Fig. 3 (a–c) FESEM and (d–f) EDAX analysis for RH, HC, and BC.

Fig. 4 (a–c) FESEM and (d–f) EDAX analysis for ZVI@RH, ZVI@HC, and ZVI@BC.

3.4. XPS analysis

The elemental composition of the surfaces of ZVI@RH, ZVI@HC, and ZVI@BC was analysed using X-ray photoelectron spectroscopy (XPS). Fig. 5a–c display the full XPS survey scans of ZVI@RH, ZVI@HC, and ZVI@BC. The main elements found on their surfaces were Fe 2p, carbon, nitrogen, oxygen, and silica. The peaks for these elements ranged from 704 eV to 736 eV for Fe 2p orbitals,²³ 284.8 eV for C 1s orbitals,²⁴ 390–410 eV for N 1s orbitals,²⁵ 531.4 eV for O 1s orbitals,²⁶ and 98–110 eV for Si 2p orbitals.²⁷

Fig. 5 X-ray photoelectron spectroscopy survey scans for (a) ZVI@RH, (b) ZVI@HC, and (c) ZVI@BC; (d) Fe 2p spectra for ZVI@RH; (e) Fe 2p spectra for ZVI@HC; and (f) Fe 2p spectra for ZVI@BC.

The Fe 2p_3/2 binding energy peaks at 711.6 eV for ZVI@RH; 710.4, 711.2, and 712.8 eV for ZVI@HC; and 711.2 and 713.1 eV for ZVI@BC; and Fe 2p_1/2 binding energy peaks at 724.8 eV for ZVI@RH; 724.5 eV for ZVI@HC; and 724.8 eV for ZVI@BC correspond to the presence of zerovalent iron (Fe⁰) in the samples (Fig. 5d–f). However, a diminished peak at 731.3 eV indicates the presence of iron oxide as a passivation layer on the surface of nanocomposites.²⁷

The C 1s spectrum of ZVI@RH, ZVI@HC, and ZVI@BC derived from lignocellulose biomass confirmed the presence of C–C with binding energies of 284.8 eV (Fig. S2a–c†). On the other hand, the O 1s XPS spectrum exhibits binding energy peaks at 528.1, 529.9, and 531.2 eV for ZVI@RH; 527.5, 529.6, and 531.2 eV for ZVI@HC; and 529.9, 531.2, and 532.2 eV for ZVI@BC, respectively. In O 1s, the peak at 529.9 eV is generally associated with lattice oxygen in iron oxides (i.e., FeO and Fe₂O₃)²⁸ (Fig. S2g–i†).

The peak at 531.2 eV is often attributed to oxygen in hydroxyl groups (–OH) on the surface.²⁷ The XPS spectrum of N 1s exhibits binding energy peaks at 400.5 eV, 399.2 eV, and 398.9 eV for ZVI@RH; 399.2 and 398.7 eV for ZVI@HC; and 407.8, 403.1, 400.1, and 399.9 eV for ZVI@BC and provides detailed insights into the different nitrogen species on the surface of nanocomposites (Fig. S2d–f†). The peak at 407.8 eV is typically attributed to oxidized nitrogen species such as nitrogen in amides (–CONH₂).²⁹ The peak at 403.1 eV corresponds to CN_x compounds and does not correspond to any form of oxidized nitrogen.³⁰ The peak at 400.5 and 399.9 eV is often associated with amines (–NH₂) or pyrrolic nitrogen, which are common in organic materials and charred products. The peak at 398.7 eV is due to pyridinic nitrogen in nanocomposites.²⁹

The XPS spectrum of Si 2p shows peaks at 102.3 eV, 103.8 eV, 104.5 eV and 105.2 eV for ZVI@RH; 102.3 eV, 103.9 eV, 104.8 eV and 105.4 eV for ZVI@HC; and 101.9 eV, 102.8 eV, and 103.8 eV for ZVI@BC. The peak at 102.3 eV is typically associated with the Si–O/Si–O–C bond.²⁶ The peak at 103.8 eV corresponds to silicon dioxide (SiO₂), indicating the presence of fully oxidized silicon.³¹ The peak at 104.5 eV and 105.4 eV can be attributed to silicon in higher oxidation states or more complex siloxane networks (Si–O–Si), reflecting additional oxygen coordination³¹ (Fig. S2j–l†).

4. Optimization of experimental parameters for OTC removal

Experimental variables like pH levels, nanocomposite material dosage, initial dye concentration, and PMS concentration can significantly influence the chemical and physical characteristics of OTC in nanocomposite-mediated removal processes. Therefore, preliminary experiments were conducted to identify the most effective operational parameters before initiating batch studies.

4.1. Effect of pH

The pH of the solution significantly affects the removal of oxytetracycline (OTC) using rice husk biomass (RH) and its derived carbonaceous materials including hydrochar (HC) and biochar (BC). The pH determines the surface charge of the adsorbate and the adsorbent molecules. The removal efficiency of OTC molecules is pH-dependent and affects their interaction with the surface functional groups of the adsorbents. Under acidic conditions, the OTC molecules tend to be positively charged owing to the protonation of amino groups, while in an alkaline medium, they become negatively charged.³² The pH_zpc of OTC was observed to be 5.5, confirming a positive charge on the OTC surface at a pH below 5.5.² The surface charge of the biomass derivatives changes with pH, influencing electrostatic interactions with OTC molecules. At low pH, the surface of biomass becomes positively charged creating repulsive interactions with positively charged OTC molecules. On the other hand, at higher pH, the surface becomes negatively charged, favouring electrostatic interaction between OTC and the adsorbent surface. The point of zero charge (pH_zpc) values for RH, HC, and BC³³ were 7.9, 7.5, and 3.38, respectively, indicating that their surface becomes positively charged at pH levels below their pH_zpc (Fig. S1†). In an acidic environment pH ≤ 3, the OTC removal efficiencies of RH and HC were 15.7% and 19.5%, and were slightly increased to 20.8% and 28.3%, respectively, with the increase in pH from 3 to 9. Conversely, BC exhibited 47.9% OTC removal efficiency at pH 3, gradually decreasing with increasing pH values.

The nanocomposites with immobilized zerovalent iron (ZVI@RH, ZVI@HC, and ZVI@BC) demonstrate dual mechanisms for removing OTC across various pH levels (Fig. 6b). These composites adsorb the OTC molecule and also facilitate a Fenton-like process, generating reactive oxygen species for its degradation. The removal efficiencies of OTC with ZVI@RH, ZVI@HC, and ZVI@BC were 56.5%, 57.9%, and 72.4%, respectively, suggesting that ZVI facilitates a Fenton-like process that results in the production of hydroxyl radicals (OH˙) and degradation of OTC.¹ With a gradual increase in pH, the removal efficiencies of ZVI@RH, ZVI@HC, and ZVI@BC composites gradually decreased. At alkaline pH (i.e. 9), the removal efficiencies of OTC with ZVI@RH, ZVI@HC, and ZVI@BC remain at 34.3%, 40.2%, and 58.9%, indicating that a negatively charged surface of the adsorbent biomass has a synergistic impact on the adsorption efficiencies of OTC. Alkaline pH also inhibits the generation of hydroxyl radicals from Fenton-like processes, leading to an overall reduced degradation efficiency of OTC.

Fig. 6 Effect of variation in pH of the solution on the OTC (50 mg L⁻¹) removal efficiencies (a) with RH, HC, and BC; (b) with ZVI@RH, ZVI@HC, and ZVI@BC; (c) with RH-PMS, HC-PMS, and BC-PMS activation; (d) with ZVI@RH-PMS, ZVI@HC-PMS, and ZVI@BC-PMS activation.

Further, to achieve maximum removal efficiency, peroxymonosulfate (PMS) was utilized as an oxidizing agent to enhance the effectiveness of the Fenton-like process for oxytetracycline (OTC) degradation. The experiment was performed using a 100 mL solution of OTC (50 mg L⁻¹) with the addition of RH (300 mg L⁻¹) and PMS concentration (300 mg L⁻¹), with the pH ranging from 2 to 9. The presence of silica on the surfaces of rice husk can activate PMS with the generation of ROS.² XPS suggests that Si present in a high oxidation state receives an electron from PMS to generate Si (δ−1⁺), and the accelerating Si (δ⁺)/Si (δ−1⁺) cycle might be beneficial to PMS activation, which leads to the formation of an intermediate of O₂˙⁻ (eqn (1)). In the absence of background anions, the self-combination of O₂˙⁻ results in the generation of ¹O₂ (eqn (2)), analogous to the Haber Weiss reaction.³⁴

O=Si(δ⁺) + 2HSO₅⁻ + H₂O → O=Si(δ−1⁺) + O₂˙⁻ + SO₄²⁻ + 4H⁺ + SO₅²⁻ (1)

O₂˙⁻ + 2H₂O → ¹O₂ + H₂O₂ + 2OH⁻ (2)

The maximum removal efficiency (50.6%) was observed at pH 2 in the RH-PMS process. A further increase in the pH up to 9 decreases the removal efficiency to 30.2%. Biochar and hydrochar materials derived from biomass pyrolysis and hydrothermal treatment contain aromatic moieties with delocalized electrons, which can serve as sites for generation and stabilization of persistent free radicals. In HC-PMS and BC-PMS processes, the presence of persistent free radicals (PFRs) on hydrochar (HC) and biochar (BC) surfaces plays a crucial role in the activation of peroxymonosulfate (PMS) for oxytetracycline (OTC) removal. The transfer of persistent free radicals from the hydrochar/biochar surface can initiate the activation of PMS. These persistent free radicals can abstract electrons from PMS, forming sulfate radicals (SO₄˙⁻) and hydroxyl radicals (˙OH).³⁵ These sulfate radicals are highly reactive and play a key role in oxidizing OTC molecules into less harmful or non-toxic by-products. Further, biochar obtained via pyrolysis (600 °C) may exhibit more structural oxygen defects, and act as an electron conductor for activation of molecular oxygen via non-radical routes. The experiment results suggest that maximum OTC removal efficiencies of 55.3% and 62.1% in HC-PMS and BC-PMS processes, respectively, were observed at pH 2 (Fig. 6c).

Further, the influence of pH on OTC removal using zerovalent iron immobilized nanocomposites (ZVI@RH, ZVI@HC, and ZVI@BC) was also investigated. The leaching of Fe²⁺ ions from nanocomposites (ZVI@RH, ZVI@HC, or ZVI@BC) triggers the activation of PMS, leading to the generation of reactive oxygen species (ROS) such as sulfate radicals (SO₄˙⁻), hydroxyl radicals (˙OH), superoxide (O₂˙⁻), singlet oxygen (¹O₂), etc., as illustrated in eqn (3)–(9).^36–39 These ROS possess potent oxidative capabilities that enhance the degradation of OTC molecules. Experimental results indicate that ZVI@BC (98.3%) exhibits superior OTC removal efficiency compared to ZVI@HC (82.5%), and ZVI@RH (79.8%) at pH ≤ 3, as depicted Fig. 6d. This higher efficiency is attributed to the synergistic effects involving lignocellulosic biomass, PMS, and zerovalent iron. The lignocellulosic biomass adsorbs OTC onto its surface near the reactive zerovalent iron which activates PMS to generate ROS, facilitating the degradation of the adsorbed OTC.¹⁰ The influence of various biomass supports on zerovalent iron (ZVI) for OTC removal shows that ZVI@BC achieves higher degradation rates (98.3%), attributed to BC, and has superior adsorption and electron transfer efficiencies to hydrochar (HC) and rice husk powder (RH), as depicted in Fig. 6d. The slight decrease in removal efficiency from pH 3 to 9 indicates that the catalyst effectively operates across a broad pH range. These findings suggest that the combination of PMS with a Fenton-like process mitigates pH-dependent limitations, demonstrating effective removal efficiency over a wide pH range. Unlike traditional Fenton reagents, PMS activation shows less sensitivity to pH changes, highlighting its potential as a promising strategy to enhance the Fenton-like process for OTC degradation.

Formation of reactive oxygen species (ROS).
Generation of sulfate radicals.

Fe⁰ + O₂ + H⁺ → Fe²⁺ + H₂O₂ (3)

Fe²⁺ + HSO₅⁻ → Fe³⁺ + SO₄˙⁻ + OH⁻ (4)

Generation of hydroxyl radicals.

Fe²⁺ + H₂O₂ → Fe³⁺ + ˙OH + OH⁻ (5)

SO₄˙⁻ + H₂O → SO₄²⁻ + ˙OH + H⁺ (6)

Generation of superoxide radicals.

˙OH + H₂O₂ → H⁺ + O₂˙⁻ + H₂O (7)

Generation of singlet oxygen species.

2O₂˙⁻ + 2H⁺ → H₂O₂ + ¹O₂ (8)

O₂˙⁻ + ˙OH → ¹O₂ + OH⁻ (9)

4.2. Effect of catalyst dose

The ZVI@RH, ZVI@HC, and ZVI@BC nanocomposite dosage affects the OTC degradation. In order to assess the impact of catalyst dosage on OTC removal using the Fenton-like process with PMS activation, an investigation was conducted involving an OTC concentration of 50 mg L⁻¹, a PMS dosage of 300 mg L⁻¹, and catalyst dosages ranging from 100 to 500 mg L⁻¹. The experimental results indicate that increasing the catalyst dose from 100 to 300 mg L⁻¹ significantly improves the OTC removal efficiencies of various nanocomposites. As the catalyst dose was increased in the ZVI-mediated PMS activation process, a large amount of Fe²⁺ ions were released in the solution, which generates more hydroxyl radicals (˙OH) in the acidic medium and also activates PMS to release ROS, which causes oxidative degradation of OTC. Nearly 79.8%, 83.9%, and 98.3% OTC removals were observed with a catalyst dose of 300 mg L⁻¹ for ZVI@RH, ZVI@HC, and ZVI@BC, respectively. However, increasing the catalyst dose beyond 300 mg L⁻¹ does not affect the removal efficiency of OTC as shown in Fig. 7. The excess Fe²⁺ ions were further oxidized to Fe³⁺ ions, which act as scavengers for PMS, generating a weaker oxidizing agent (SO₅˙⁻), as shown in eqn (7). Therefore, the optimum catalyst dose of 300 mg L⁻¹ was selected for further studies.

Fig. 7 Effect of catalyst dose on the OTC (50 mg L⁻¹) removal efficiencies of ZVI@RH-PMS, ZVI@HC-PMS, ZVI@BC-PMS {[PMS] = 300 mg L⁻¹, pH 3}.

4.3. Effect of PMS dose

The efficiency of removing OTC (50 mg L⁻¹) was investigated using various PMS concentrations (100–500 mg L⁻¹) with a catalyst dose of 300 mg L⁻¹ at pH 3, as illustrated in Fig. 8. When PMS was introduced at a concentration of 100 mg L⁻¹, the removal efficiencies of OTC were measured at 50.8%, 58.2%, and 76.8% for ZVI@RH, ZVI@HC, and ZVI@BC, respectively. This was attributed to the activation of PMS, generating both radical and non-radical reactive oxygen species (ROS), including sulfate radicals (SO₄˙⁻), hydroxyl radicals (˙OH), superoxide radicals (O₂˙⁻), and singlet oxygen (¹O₂). Upon increasing the PMS concentration to 300 mg L⁻¹, the removal efficiencies improved to 79.8%, 83.9%, and 98.3% for ZVI@RH, ZVI@HC, and ZVI@BC, respectively. The higher PMS concentration led to increased production of reactive oxygen species (ROS), thereby enhancing the overall degradation rate. However, further increasing the PMS dose beyond 300 mg L⁻¹ resulted in decreased OTC removal efficiency.

Fig. 8 Effect of PMS dose on the OTC (50 mg L⁻¹) removal efficiencies of ZVI@RH-PMS, ZVI@HC-PMS, ZVI@BC-PMS.

These findings suggest that at higher PMS concentrations, there is a tendency for excess PMS to scavenge sulfate radicals (SO₄˙⁻) and hydroxyl radicals (˙OH) through reactions detailed in eqn (10)–(14).⁴⁰ This excess PMS can generate a weaker oxidant known as the peroxymonosulfate radical (SO₅˙⁻), which may diminish the oxidation kinetics and efficiency of the process, leading to marginal improvements in OTC removal efficiencies. Therefore, optimizing the PMS dosage is crucial to maximize ROS generation for efficient OTC degradation, rather than allowing excessive PMS consumption through self-quenching reactions.

Scavenging of reactive oxygen species (ROS).

Fe³⁺ + HSO₅⁻ → Fe²⁺ + SO₅˙⁻ + H⁺ (10)

HSO₅⁻ + SO₄˙⁻ → SO₅˙⁻ + SO₄²⁻ + H⁺ (11)

HSO₅⁻ + ˙OH → SO₅˙⁻ + H₂O (12)

SO₄˙⁻ + SO₄˙⁻ → S₂O₈²⁻ (13)

SO₅˙⁻ + SO₅˙⁻ → S₂O₈²⁻ + O₂ (14)

4.4. Effect of initial OTC concentrations

To evaluate the impact of different activation processes on OTC removal efficiency, we examined the effect of varying initial OTC concentrations from 20 to 160 mg L⁻¹, while maintaining a constant PMS concentration of 300 mg L⁻¹ and a catalytic load of 300 mg L⁻¹ at pH 3 (Fig. 9a–c). It was observed that at lower OTC concentrations (50 mg L⁻¹), the removal efficiencies were maximized, indicating a higher reaction rate, which decreased notably as the concentration increased beyond 50 mg L⁻¹. The lower OTC concentrations likely facilitate faster degradation processes due to the greater availability of reactive oxygen species (such as sulfate and hydroxyl radicals) relative to the concentration of OTC molecules. This abundance of reactive species promotes more frequent collisions between OTC molecules and reactive species, thereby accelerating degradation rates. Conversely, as the OTC concentration rises, the availability of reactive species may become insufficient, resulting in slower degradation kinetics. Therefore, an initial OTC concentration of 50 mg L⁻¹ was identified as optimal for further investigation, as it demonstrated the highest removal efficiencies under conditions of 300 mg L⁻¹ catalytic dose and 300 mg L⁻¹ PMS concentration.

Fig. 9 Effect of initial OTC concentrations on removal efficiencies of OTC with (a) ZVI@RH-PMS, (b) ZVI@HC-PMS, (c) ZVI@BC-PMS processes.

4.5. Control experiment

The control experiments were conducted to investigate the role of rice husk and its derived carbonaceous materials, as well as its nanocomposites (ZVI@RH, ZVI@HC, and ZVI@BC), in the PMS activation process and their impact on the removal efficiencies of oxytetracycline. The OTC stability (at a concentration of 50 mg L⁻¹) was assessed through the removal studies conducted under experimental conditions at pH 3, both in the presence and absence of PMS (300 mg L⁻¹) as an external oxidizing agent. For this, the concentration of OTC showed insignificant change, indicating the inherent stability of OTC.

To assess the effectiveness of rice husk powder (RH) and its derived carbon materials (HC and BC) for the removal of OTC, a 100 mL solution containing OTC (50 mg L⁻¹) at pH 3 was treated with a 300 mg L⁻¹ dose of RH, HC, and BC for 20 min. The UV-vis spectrum was recorded to measure the remaining OTC concentration in the solution. OTC displayed insignificant change in its concentration, indicating the inherent stability of OTC under the experimental conditions. The adsorption efficiencies of OTC were found to be 19.5% for rice husk (RH), 23.3% for hydrochar (HC), and 57.9% for biochar (BC). These results suggest that modifying rice husk through hydrothermal treatment (HC) and carbonization (BC) enhances OTC adsorption by altering functional groups and creating additional pores. When rice husk (RH), hydrochar (HC), and biochar (BC) immobilized ZVI nanoparticles (ZVI@RH, ZVI@HC, and ZVI@BC nanocomposites, respectively) were used to assess their OTC removal efficiencies under similar experimental conditions, it was observed that the OTC removal efficiencies for ZVI@RH, ZVI@HC, and ZVI@BC nanocomposites were enhanced to 56.5%, 58.9%, and 72.4% respectively. This increase in OTC removal efficiencies was attributed to the reaction of ZVI with dissolved O₂ under acidic conditions, triggering a Fenton-like process to generate hydroxyl radicals (˙OH), as well as an increase in the adsorption efficiency after treatment, which generated a synergistic effect responsible for the OTC degradation process (Fig. 10).

Fig. 10 Effect of 300 mg L⁻¹ dose of rice husk powder (RH); rice husk powder derived hydrochar (HC) and biochar (BC); PMS only; ZVI@RH only; ZVI@HC only; and ZVI@BC only on the removal of aqueous OTC (50 mg L⁻¹) at pH 3.

The PMS activation process alongside the inherent Fenton-like process given by ZVI@RH, ZVI@HC, and ZVI@BC nanocomposites, significantly improved the oxidative degradation of OTC to 79.8%, 83.9%, and 98.3%, respectively. The higher OTC removal efficiencies for the ZVI@RH-PMS, ZVI@HC-PMS, and ZVI@BC-PMS processes were due to advanced oxidation processes. The ROS generated from ZVI-mediated PMS activation via a Fenton-like process played a key role in the OTC degradation. The increased OTC removal efficiencies for the ZVI@RH-PMS, ZVI@HC-PMS, and ZVI@BC-PMS processes can also be attributed to the synergistic process. The rice husk, hydrochar, and biochar first brought the OTC molecule near the reactive sites on the catalytic surface through their higher adsorption efficacies, and ZVI activated PMS to generate reactive oxygen species. The maximum degradation of OTC was observed with ZVI@BC, indicating that after modification of rice husk, the produced biochar has higher adsorption efficiency. Thus, in the case of the ZVI@BC-PMS process, biochar has a stronger affinity for attracting the OTC molecule toward the reactive sites of the catalyst compared to rice husk (RH) and hydrochar (HC) (Fig. 11). The UV-vis spectrum of an aqueous OTC solution exhibits an absorption peak at 357 nm. Upon adding ZVI@BC (300 mg L⁻¹) and PMS (300 mg L⁻¹) to the solution, the absorption peak shifts to 330 nm, accompanied by a decrease in absorption intensity with 20 min of reaction time (Fig. 12).

Fig. 11 Effect of various catalyst doses on the PMS activation process mediated degradation of OTC.

Fig. 12 Changes in the UV-vis spectrum of OTC (50 mg L⁻¹) using the ZVI@BC-PMS activation process.

To ascertain the molar mass of dye degraded per mole of PMS, we also computed the PMS utilization efficiency (η_PMS) for ZVI@RH-PMS, ZVI@HC-PMS, and ZVI@BC-PMS. According to the findings (Fig. S3†), ZVI@BC demonstrated superior PMS utilization efficiencies at 0.942 mM compared to 0.845 mM and 0.882 mM for ZVI@HC and ZVI@RH, respectively. Degradation kinetics refers to the study of the rates at which OTC degradation occurs with ZVI@RH-PMS, ZVI@HC-PMS, and ZVI@BC-PMS catalysts (Fig. S4†). The observed rate constant (K_obs) for ZVI@BC (0.82 min⁻¹) is significantly higher than that for ZVI@RH (0.69 min⁻¹) and ZVI@HC (0.72 min⁻¹). This indicates that ZVI@BC exhibits superior degradation kinetics. The higher K_obs suggest that the biochar matrix provides a more effective environment for the catalytic activities of the zerovalent iron particles, possibly due to enhanced surface area, better dispersion of iron particles, or improved electron transfer processes. In contrast, the rice husk and hydrochar matrices might not facilitate the same level of interaction or may have less optimal structural properties, leading to lower observed rate constants (Table 2). The rice husk-derived carbon supports offer a sustainable and cost-effective alternative to traditional carbon supports used in advanced oxidation processes. These materials, being biogenic and derived from abundant agricultural waste, not only provide an eco-friendly solution but also exhibit remarkable surface properties that enhance the catalytic activity of biogenic Fe⁰. The incorporation of rice husk carbon supports amplifies the efficiency of the reaction by facilitating the generation and stabilization of reactive species, thus promoting faster and more efficient degradation of pollutants such as oxytetracycline.

Table 2 OTC degradation kinetics of ZVI@RH-PMS, ZVI@HC-PMS, and ZVI@BC-PMS activation processes

Kinetic parameters	ZVI@RH	ZVI@HC	ZVI@BC
K obs. (min^−1)	6.9 × 10^−1	7.2 × 10^−1	8.2 × 10^−1
R ^2	0.9905	0.9912	0.9949
R ^2adj.	0.9874	0.9882	0.9933
χ ^2	2.25	1.58	1.93

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This study held significant environmental importance as it addressed the pressing need for sustainable and effective methods to remediate pharmaceutical pollutants in water systems. By systematically evaluating rice husk (RH) and its carbonaceous derivatives—hydrochar (HC) and biochar (BC)—as supports for biogenic zerovalent iron (ZVI) in advanced oxidation processes (AOPs), this research highlighted the potential of utilizing agricultural waste for environmental remediation. Among the materials studied, biochar (BC) demonstrated superior performance in oxytetracycline (OTC) removal due to its enhanced surface properties from pyrolysis treatment, which promoted both adsorption and the activation of peroxymonosulfate (PMS). The synergistic effect in the ZVI@BC-PMS system, characterized by efficient reactive oxygen species (ROS) generation, enabled near-complete OTC degradation. This approach not only provided a sustainable use for agricultural waste but also contributed to the development of greener, more efficient AOPs for mitigating the environmental impact of pharmaceutical contaminants.

5. Possible mechanism

The effectiveness of peroxymonosulfate in OTC degradation depends on the specific active species involved. To investigate the role of different reactive oxygen species involved in the OTC degradation process via ZVI@RH, ZVI@HC, and ZVI@BC mediated PMS activation, the experiments were performed in the presence of various radical scavengers such as ethanol (EtOH), tert-butyl alcohol (^tBuOH), urea, L-histidine, etc. Ethanol efficiently scavenges both SO₄˙⁻ (7.7 × 10⁷ M⁻¹ s⁻¹) and ˙OH (2.8 × 10⁹ M⁻¹ s⁻¹) radicals.⁴¹ In contrast, tert-butyl alcohol (^tBuOH) demonstrates higher efficiency in scavenging ˙OH (7.6 × 10⁸ M⁻¹ s⁻¹) compared to SO₄˙⁻ (9.1 × 10⁵ M⁻¹ s⁻¹).⁴² Urea⁴³ exhibits selectivity towards O₂˙⁻ while L-histidine shows a preference for ¹O₂ scavenging.⁴⁴ The observed order of OTC degradation inhibition by various radical scavengers, from highest to lowest, was found to be: L-histidine > urea > ^tButOH > EtOH, as depicted in Fig. 13.

Fig. 13 Inhibition effect of various scavenging agents, viz. EtOH, ^tBuOH, urea, and L-histidine on the OTC removal efficiencies of (a) ZVI@RH-PMS; (b) ZVI@HC-PMS; (c) ZVI@BC-PMS processes.

The addition of ethanol (EtOH) and tertiary butanol (^tBuOH) had only a minor impact on the degradation of oxytetracycline (OTC), suggesting that hydroxyl radicals (˙OH) and sulfate radicals (SO₄˙⁻) might not be the primary active species responsible for degradation under these conditions. Conversely, urea's inhibition of the OTC degradation process suggests that superoxide radicals (O₂˙⁻) are indeed active species in the process. However, the most pronounced inhibition was observed with L-histidine, indicating that singlet oxygen (¹O₂) is the predominant reactive oxygen species involved in the OTC degradation process (Table 3).

Table 3 Effect of various radical scavengers on ZVI@RH-PMS, ZVI@HC-PMS, and ZVI@BC-PMS activation processes for OTC removal

Radical scavengers	Conc. (mM)	ZVI@RH-PMS process	ZVI@HC-PMS process	ZVI@BC-PMS process
No scavenger	—	79.8%	83.9%	98.3%
Ethanol	50	27.8%	37.3%	63.4%
Urea	3.2	19.8%	26.7%	46.3%
L-Histidine	1.5	15.4%	15.7%	23.9%
Tert-butyl alcohol	3.2	23.7%	33.2%	53.6%

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5.1. Density functional theory (DFT)

The density functional theory (DFT) analysis of oxytetracycline (OTC) providing chemical potential (μ), electrophilicity index (ω), and chemical hardness (eqn (S17)†) gives valuable insights into its reactivity and interaction with reactive oxygen species (ROS). The molecular electrostatic potential map of OTC molecules has electron-rich (red) and electron-poor (green) regions indicating electron charge density distribution within the molecule. The highest occupied molecular orbitals (HOMOs) are mainly located on the electron-dense aromatic rings, making them vulnerable to attacks by electrophilic ROS. The lower energy values of the HOMO (E_H = −5.92 eV) and LUMO (E_L = −2.18 eV), electrophilicity index (ω = 4.378 eV), and chemical potential (μ = 4.051 eV) describe the electron-accepting nature of the OTC molecule. The lower value of the chemical hardness index (η = 1.87 eV) signifies the lower stability of the OTC molecule.⁴⁵ Further, the lower value of the HOMO–LUMO energy gap (ΔE = 3.74 eV) corresponds to the higher reactivity of OTC molecules. The higher Fukui indices [Fukui(–)] at C-4, C-7, C-8, N-2, and [Fukui(0)] at C-19 and N-2, point to the molecular sites most likely to be targeted by electrophilic ROS. This suggests that degradation of OTC by ROS will primarily occur at these locations, leading to the breakdown of the molecular structure (Fig. 14).

Fig. 14 (a) Normal and (b) optimized structure of OTC; (c) contour plots representing HOMOs, and (d) LUMOs of OTC; (e) Fukui indices of OTC.

5.2. High-resolution mass spectrometry

High-resolution mass spectrometry (HRMS) analysis of oxytetracycline (OTC) revealed a detailed degradation pathway in the presence of the ZVI@BC-PMS process (Fig. 15 and S5–S7†). OTC exhibited a peak at m/z 460.1 corresponding to its molecular mass. The molecular mass peak disappeared with the addition of catalyst and PMS within 10 min of the reaction time, and the formation of other peaks corresponds to the degradation intermediates or by-products. In total, eleven degradation intermediates including 1a–1f and 2a–2e were detected. The mass peak at m/z 425.7 [M + H]⁺ corresponds to intermediate 1a obtained through radical mediated dehydroxylation and removal of hydrogen from OTC molecules. The mass peak at m/z 317.2 corresponds to intermediate 1b obtained after deamination, deamidation, and dehydroxylation processes. Further, the intermediate 1b underwent mineralization through two degradation pathways: (a) via intermediate 1c (m/z 274.2) obtained after dehydroxylation and demethylation of 1b; and (b) via intermediate 2a (m/z 301.2) obtained after dehydroxylation of 1b. The intermediate 2a through 2b [M + H]⁺ (m/z 279.1) further generates two intermediates 1d (m/z 190.1) after demethylation and ring opening, and intermediate 2c (m/z 214) after dehydroxylation of 2b. The ring opening of 2c gives 2d and 2e intermediates with mass peaks at m/z 122 and m/z 106, respectively, and ultimately results in mineralized products. The intermediates 1c and 1d are converted into intermediate 1e (m/z 160.2) after various ring opening and decarbonylation steps. The radical mediated degradation of 1e leads to mineralized products through the formation of intermediate 1f [M + H]⁺ (m/z 149).

Fig. 15 Proposed degradation pathway for OTC by the ZVI@BC-PMS process.

6. Water quality parameters

It is important to consider the potential environmental impact of the by-products resulting from the oxidation of dyes during oxidative degradation. Therefore, it is essential to establish qualitative parameters to manage water pollution effectively. The American Public Health Association (APHA) recommends using standard analytical techniques such as chemical oxygen demand (COD), biological oxygen demand (BOD) and total organic carbon (TOC) to assess the organic content in wastewater and treatment processes.⁴⁶ The data in Table 4. show a significant reduction in COD, BOD and TOC values after the ZVI@RH-PMS, ZVI@HC-PMS, and ZVI@BC-PMS processes, indicating that the by-products generated after OTC degradation are not harmful. The TOC reduction in OTC was 67.5%, 77.4% and 63.2% with PMS activation of heterogeneous catalysts ZVI@RH-PMS, ZVI@HC-PMS and ZVC@BC-PMS, respectively.

Table 4 Water quality parameters for ZVI@RH, ZVI@HC, and ZVI@BC mediated PMS activation and OTC degradation processes

Water quality parameter	Initial values (mg L^−1)	ZVI@RH		ZVI@HC		ZVI@BC
		(mg L^−1)	Removal efficiency (%)	(mg L^−1)	Removal efficiency (%)	(mg L^−1)	Removal efficiency (%)
COD	1092	482	55.8	353	67.6	293	73.1
BOD	392	162	58.6	138	64.7	106	72.9
TOC	35.5	11.54	67.5	8.037	77.4	13.08	63.2

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7. Regeneration and reusability of ZVI@RH, ZVI@HC, and ZVI@BC

Fig. S8† illustrates the reusability of ZVI@RH, ZVI@HC, and ZVI@BC in PMS activation and degradation of OTC. The passivation layer formation or catalytic poisoning by mineralized by-products slightly reduces the surface activity for PMS activation and OTC degradation with each regeneration cycle. This poisoning effect impedes the release of Fe²⁺ ions into the solution, which is essential for effective PMS activation. To evaluate the durability of the ZVI@RH, ZVI@HC, and ZVI@BC catalysts, we analysed their FESEM, EDAX, and XRD spectra before and after usage. The results indicated minimal corrosion on the catalyst surfaces during OTC degradation, as evidenced by FESEM images and EDAX analysis (Fig. S9a–f†). Furthermore, the XRD spectra of ZVI@RH, ZVI@HC, and ZVI@BC revealed no additional peaks, suggesting that no metal oxides formed during the PMS activation process (Fig. S10a–c†). The levels of metal leaching into the solution throughout the experiment were determined using atomic absorption spectrometry (AAS). The concentrations of dissolved Fe in the solution were found to be below the detection limit (BDL) for ZVI@RH, 8.75 μg L⁻¹ for ZVI@HC, and 27.57 μg L⁻¹ for ZVI@BC. These concentrations are within the acceptable limits specified by the United States Environmental Protection Agency (USEPA 1985) for maintaining drinking water quality.⁴⁷

8. Effect of interfering anions

The effectiveness of ZVI@RH-PMS, ZVI@HC-PMS, and ZVI@BC-PMS activation processes for OTC removal was investigated in the presence of interfering anion concentrations (30 mM) such as chloride (Cl⁻), nitrate (NO₃⁻), carbonate (CO₃²⁻), and dihydrogen phosphate (H₂PO₄⁻) in water systems (Fig. 16). The chloride ion (Cl⁻) has been found to decrease the efficiency of removing OTC by reacting with sulfate radicals (SO₄˙⁻) and hydroxyl radicals (˙OH) in the ZVI@RH-PMS, ZVI@HC-PMS, and ZVI@BC-PMS activation processes. This results in the formation of less reactive chlorine species such as Cl˙ and Cl₂˙⁻ (eqn (15)). Similarly, the presence of nitrate anions (NO₃⁻) reduces the removal efficiencies of OTC by impeding the Fenton degradation processes and replacing SO₄˙⁻ and hydroxyl radicals (˙OH) radicals with less reactive radicals such as NO₃˙ and especially NO₂˙ (eqn (16)–(18)). On the other hand, the presence of carbonate anions (CO₃²⁻) can also scavenge SO₄˙⁻ and ˙OH radicals, producing the less reactive carbonate radical (CO₃˙⁻) (eqn (19)). Further, dihydrogen phosphate anions (H₂PO₄⁻) effectively inhibit the activation of PMS and the degradation of OTC due to their ability to bind to the catalyst's surface, and their capacity to form complexes with Fe²⁺ ions (eqn (20) and (21)).

Cl˙ + Cl⁻ → Cl₂˙⁻ (15)

NO₃⁻ + SO₄˙⁻ → NO₃˙ + SO₄²⁻ (16)

NO₂⁻ + SO₄˙⁻ → NO₂˙ + SO₄²⁻ (17)

NO₃⁻ + ˙OH → NO₃˙ + OH⁻ (18)

CO₃²⁻ + SO₄˙⁻ → CO₃˙⁻ + SO₄²⁻ (19)

SO₄˙⁻ + H₂PO₄⁻ → H₂PO₄˙⁻ + SO₄²⁻ (20)

OH˙ + H₂PO₄⁻ → H₂PO₄˙⁻ + OH⁻ (21)

Fig. 16 Effect of interfering anions (Cl⁻, NO₃⁻, CO₃²⁻, H₂PO₄⁻) on the OTC removal efficiencies of ZVI@RH-PMS; ZVI@HC-PMS and ZVI@BC-PMS processes.

9. Conclusion

In this study, we systematically evaluated the effects of rice husk (RH) and its derived carbonaceous materials i.e. hydrochar (HC) and biochar (BC), as supports for biogenic zerovalent iron ZVI@RH, ZVI@HC, and ZVI@BC nanocomposites. These materials were tested in advanced oxidation processes (AOPs) for the removal of oxytetracycline (OTC). Among the supports investigated, biochar (BC) showed superior OTC removal performance due to its enhanced surface properties resulting from pyrolysis treatment. BC also demonstrated better activation of peroxymonosulfate (PMS) for OTC degradation compared to RH and HC. The ZVI@BC nanocomposite achieved higher OTC removal efficiencies in heterogeneous Fenton-like processes compared to ZVI@RH and ZVI@HC. The addition of PMS further improved OTC removal by generating higher levels of reactive oxygen species (ROS), leading to more effective degradation compared to the Fenton process alone and pushing the OTC degradation process closer to completion. The increased surface defects in BC associated with pyrolysis treatment contributed to both greater OTC adsorption and degradation, and also enhanced the ZVI-mediated activation of PMS in ZVI@BC, creating a synergistic effect that nearly completed OTC removal from the aqueous medium. Additionally, the performance of the ZVI@BC-PMS activation process was found to be comparable to other iron-mediated AOPs for OTC removal (Table 5). The radical scavenger analysis and density functional theory (DFT)-based structural optimization provide new insights into the mechanistic pathways of OTC degradation, supported by HRMS-based intermediate identification. The findings not only showcase the efficiency of ZVI@BC in heterogeneous Fenton-like processes but also suggest its scalability for practical applications. These insights lay the groundwork for tailoring biochar-based materials for broader applications in environmental remediation. The use of ZVI@BC composites could be extended beyond antibiotic removal to other environmental contaminants, such as heavy metals or organic pollutants, further enhancing their practical applicability in addressing a range of environmental pollution challenges. The utilization of rice husk-derived materials as supports for biogenic zerovalent iron (ZVI) nanocomposites represents a transformative approach to sustainable environmental remediation.

Table 5 Comparison of OTC degradation via advanced oxidation processes involving the Fe-mediated advanced oxidation process

Catalyst	Contaminant (mg L^−1)	Catalyst (mg L^−1)	Oxidizing agent	Main ROS	Time (min)	pH	Removal (%)
Fe3O4@G (ref. 48)	10	60	—	—	50	7	98.2
Fe–N@BC (ref. 4)	10	100	PS	^1O2	20	7	98
Fe–Ti@CN (ref. 49)	10	100	H2O2	˙OH, ˙O2^− and ^1O2	10	4.8	90
Fe3O4/BC (ref. 50)	20	400	PMS	˙OH	120	3.8	68
Fe-oxide@BC (ref. 51)	50	500	PDS	^1O2	120	7	91
CuFe2O4 (ref. 52)	10	100	PMS	^1O2, ˙OH	25	6.8	99.6
C–BN@CoFe2O4 (ref. 53)	20	400	PMS	SO4˙^− and ^1O2	6.68	5	92.7
ZVI@RH	50	300	PMS	^1 O 2	10	3	79.8
ZVI@HC	50	300	PMS	^1 O 2	10	3	82.5
ZVI@BC	50	300	PMS	^1 O 2	10	3	98.3

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Consent for publication

The authors provide their consent for the publication of this manuscript.

Data availability

The data used for this manuscript have been cited with proper references. All the data used to write the results are included in the manuscript and can be produced by the corresponding author on reasonable request.

Author contributions

Conceptualization: Sandeep Kumar; supervision: Sandeep Kumar; methodology: Sandeep Kumar, Parminder Kaur, Sandeep Kaushal; data curation: Parminder Kaur, Janpreet Singh, J. Nagendra Babu, Sunil Mittal; formal analysis: Sandeep Kumar, Parminder Kaur, Jyoti Rani; investigation: Sandeep Kumar, Parminder Kaur, Jyoti Rani, Sandeep Kaushal; software: Janpreet Singh; validation: Sandeep Kumar, Sandeep Kaushal, J. Nagendra Babu; writing – original draft: Sandeep Kumar, Parminder Kaur, Sandeep Kaushal; writing – review & editing: Sandeep Kumar, Parminder Kaur, Sandeep Kaushal. All the authors have read and agreed to the published version of the manuscript.

Conflicts of interest

The authors have no competing interests.

References

1. S. Yildiz, H. Mihçiokur and A. Olabi, Experimental study of oxytetracycline degradation using Fenton-like processes, Int. J. Environ. Sci. Technol., 2023, 20(10), 11049–11060,  DOI:10.1007/s13762-023-05099-x.
2. M. Conde-Cid, M. J. Fernández-Sanjurjo, G. Ferreira-Coelho, D. Fernández-Calviño, M. Arias-Estevez and A. Núñez-Delgado, et al. Competitive adsorption and desorption of three tetracycline antibiotics on bio-sorbent materials in binary systems, Environ. Res., 2020, 190, 110003 , https://linkinghub.elsevier.com/retrieve/pii/S0013935120309002.
3. M. Minale, A. Guadie, Y. Li, Y. Meng, X. Wang and J. Zhao, Enhanced removal of oxytetracycline antibiotics from water using manganese dioxide impregnated hydrogel composite: Adsorption behavior and oxidative degradation pathways, Chemosphere, 2021, 280, 130926 , https://linkinghub.elsevier.com/retrieve/pii/S0045653521013977.
4. H. Tian, K. Cui, S. Sun, J. Liu and M. Cui, Sequential doping of exogenous iron and nitrogen to prepare Fe-N structured biochar to enhance the activity of OTC degradation, Sep. Purif. Technol., 2023, 322, 124249 , https://linkinghub.elsevier.com/retrieve/pii/S1383586623011577.
5. M. Wu, C. Que, L. Tang, H. Xu, J. Xiang and J. Wang, et al. Distribution, fate, and risk assessment of antibiotics in five wastewater treatment plants in Shanghai, China, Environ. Sci. Pollut. Res., 2016, 3(18), 18055–18063,  DOI:10.1007/s11356-016-6946-0.
6. Y. J. Lee, J. M. Lee, C. G. Lee, S. J. Park and E. H. Jho, Photodegradation Behavior of Agricultural Antibiotic Oxytetracycline in Water, Water, 2022, 14(21), 3379 , https://www.mdpi.com/2073-4441/14/21/3379.
7. M. Afzaal, R. Nawaz, S. Hussain, M. Nadeem, M. A. Irshad and A. Irfan, et al. Removal of oxytetracycline from pharmaceutical wastewater using kappa carrageenan hydrogel, Sci. Rep., 2024, 14(1), 19687.
8. G. I. Lupu, C. Orbeci, L. Bobirică, C. Bobirică and L. F. Pascu, Key Principles of Advanced Oxidation Processes: A Systematic Analysis of Current and Future Perspectives of the Removal of Antibiotics from Wastewater, Catalysts, 2023, 13(9), 1280 , https://www.mdpi.com/2073-4344/13/9/1280.
9. P. Kaur, S. Kumar, J. Rani, J. Singh, S. Kaushal and K. Hussain, et al. Rationally tailored synergy between adsorption efficiency of cotton shell activated carbon and PMS activation via biogenic Fe0 or Cu0 for effective mitigation of triphenylmethane dyes, Sep. Purif. Technol., 2024, 342, 127010 , https://linkinghub.elsevier.com/retrieve/pii/S1383586624007494.
10. S. Kumar, R. S. Brar, J. N. Babu, A. Dahiya, S. Saha and A. Kumar, Synergistic effect of pistachio shell powder and nano-zerovalent copper for chromium remediation from aqueous solution, Environ. Sci. Pollut. Res., 2021, 28(44), 63422–63436,  DOI:10.1007/s11356-021-15285-4.
11. S. Kumar, R. S. Brar, S. Saha, A. Dahiya, W. Kalpana and J. N. Babu, Synergistic effect of eco-friendly pistachio shell biomass on nano-MnO₂ for crystal violet removal: kinetic and equilibrium studies, Int. J. Environ. Sci. Technol., 2023, 20(5), 5123–5140,  DOI:10.1007/s13762-022-04212-w.
12. H. Tang, J. Wang, S. Zhang, H. Pang, X. Wang and Z. Chen, et al. Recent advances in nanoscale zero-valent iron-based materials: Characteristics, environmental remediation and challenges, J. Cleaner Prod., 2021, 319, 128641 , https://linkinghub.elsevier.com/retrieve/pii/S0959652621028444.
13. P. Kaur, K. Hussain, A. Kumar, J. Singh, J. N. Babu and S. Kumar, Evaluation of synergistic adsorption approach for terbinafine removal by cotton shell powder immobilized zerovalent copper: Adsorption kinetics and DFT simulation, Environ. Nanotechnol., Monit. Manage., 2023, 20, 100875 , https://linkinghub.elsevier.com/retrieve/pii/S2215153223000995.
14. P. Kaur, A. Kumar, J. N. Babu and S. Kumar, Tetracycline removal via three-way synergy between pistachio shell powder, zerovalent copper or iron, and peroxymonosulfate activation, J. Hazard. Mater. Adv., 2023, 12, 100385 , https://linkinghub.elsevier.com/retrieve/pii/S2772416623001560.
15. N. Samsalee, J. Meerasri and R. Sothornvit, Rice husk nanocellulose: Extraction by high-pressure homogenization, chemical treatments and characterization, Carbohydr. Polym. Technol. Appl., 2023, 6, 100353 , https://linkinghub.elsevier.com/retrieve/pii/S2666893923000749.
16. P. Kaur, W. Kalpana, S. Kumar, A. Kumar and A. Kumar, Effect of various sodium hydroxide treatment parameters on the adsorption efficiency of rice husk for removal of methylene blue from water, Emergent Mater., 2023, 6(6), 1809–1824,  DOI:10.1007/s42247-023-00574-0.
17. P. Malhotra, A. Jain and R. Payal, Porous Silica Nanoparticles from Rice Husk for the Elimination of Erichrome Black T (EBT) from Laboratory Waste Water, in Green Chemistry in Environmental Sustainability and Chemical Education, ed. V. S. Parmar, P. Malhotra and D. Mathur, Springer Singapore, Singapore, 2018, pp. 101–110,  DOI:10.1007/978-981-10-8390-7_10.
18. D. Touhami, Z. Zhu, W. S. Balan, J. Janaun, S. Haywood and S. Zein, Characterization of rice husk-based catalyst prepared via conventional and microwave carbonisation, J. Environ. Chem. Eng., 2017, 5(3), 2388–2394 , https://linkinghub.elsevier.com/retrieve/pii/S2213343717301550.
19. H. S. AL-Shehri, H. S. Alanazi, A. M. Shaykhayn, L. S. ALharbi, W. S. Alnafaei and A. Q. Alorabi, et al. Adsorption of Methylene Blue by Biosorption on Alkali-Treated Solanum incanum: Isotherms, Equilibrium and Mechanism, Sustainability, 2022, 14(5), 2644 , https://www.mdpi.com/2071-1050/14/5/2644.
20. S. Yefremova, A. Zharmenov, Y. Sukharnikov, L. Bunchuk, A. Kablanbekov and K. Anarbekov, et al. Rice Husk Hydrolytic Lignin Transformation in Carbonization Process, Molecules, 2019, 24(17), 3075 , https://www.mdpi.com/1420-3049/24/17/3075.
21. M. Gale, T. Nguyen, M. Moreno and K. L. Gilliard-AbdulAziz, Physiochemical Properties of Biochar and Activated Carbon from Biomass Residue: Influence of Process Conditions to Adsorbent Properties, ACS Omega, 2021, 6(15), 10224–10233,  DOI:10.1021/acsomega.1c00530.
22. I. Maamoun, R. Eljamal, O. Falyouna, K. Bensaida, Y. Sugihara and O. Eljamal, Insights into kinetics, isotherms and thermodynamics of phosphorus sorption onto nanoscale zero-valent iron, J. Mol. Liq., 2021, 328, 115402 , Available from: https://linkinghub.elsevier.com/retrieve/pii/S0167732221001288.
23. K. O. Badmus, E. Coetsee-Hugo, H. Swart and L. Petrik, Synthesis and characterisation of stable and efficient nano zero valent iron, Environ. Sci. Pollut. Res., 2018, 25(24), 23667–23684,  DOI:10.1007/s11356-018-2119-7.
24. D. Fang, F. He, J. Xie and L. Xue, Calibration of Binding Energy Positions with C1s for XPS Results, J. Wuhan Univ. Technol., Mater. Sci. Ed., 2020, 35(4), 711–718,  DOI:10.1007/s11595-020-2312-7.
25. M. Ayiania, M. Smith, A. J. R. Hensley, L. Scudiero, J. S. McEwen and M. Garcia-Perez, Deconvoluting the XPS spectra for nitrogen-doped chars: An analysis from first principles, Carbon, 2020, 162, 528–544 , https://linkinghub.elsevier.com/retrieve/pii/S0008622320302116.
26. T. J. Frankcombe and Y. Liu, Interpretation of Oxygen 1s X-ray Photoelectron Spectroscopy of ZnO, Chem. Mater., 2023, 35(14), 5468–5474,  DOI:10.1021/acs.chemmater.3c00801.
27. A. Kaur, P. Chahal and T. Hogan, Selective Fabrication of SiC/Si Diodes by Excimer Laser Under Ambient Conditions, IEEE Electron Device Lett., 2016, 37(2), 142–145 , http://ieeexplore.ieee.org/document/7355309/.
28. J. C. Wang, J. Ren, H. C. Yao, L. Zhang, J. S. Wang and S. Q. Zang, et al. Synergistic photocatalysis of Cr(VI) reduction and 4-Chlorophenol degradation over hydroxylated α-Fe₂O₃ under visible light irradiation, J. Hazard. Mater., 2016, 311, 11–19 , https://linkinghub.elsevier.com/retrieve/pii/S0304389416301820.
29. F. Li, Y. Bu, G. F. Han, H. J. Noh, S. J. Kim and I. Ahmad, et al. Identifying the structure of Zn-N2 active sites and structural activation, Nat. Commun., 2019, 10(1), 2623 , https://www.nature.com/articles/s41467-019-10622-1.
30. N. Hellgren, R. T. Haasch, S. Schmidt, L. Hultman and I. Petrov, Interpretation of X-ray photoelectron spectra of carbon-nitride thin films: New insights from in situ XPS, Carbon, 2016, 108, 242–252 , Available from: https://linkinghub.elsevier.com/retrieve/pii/S0008622316305802.
31. D. Tan, Z. Ma, B. Xu, Y. Dai, G. Ma and M. He, et al. Surface passivated silicon nanocrystals with stable luminescence synthesized by femtosecond laser ablation in solution, Phys. Chem. Chem. Phys., 2011, 13(45), 20255 , https://xlink.rsc.org/?DOI=c1cp21366k.
32. S. Jiao, S. Zheng, D. Yin, L. Wang and L. Chen, Aqueous oxytetracycline degradation and the toxicity change of degradation compounds in photoirradiation process, J. Environ. Sci., 2008, 20(7), 806–813 , https://linkinghub.elsevier.com/retrieve/pii/S1001074208621300.
33. Y. Ngernyen, D. Petsri, K. Sribanthao, K. Kongpennit, P. Pinijnam and R. Pedsakul, et al. Adsorption of the non-steroidal anti-inflammatory drug (ibuprofen) onto biochar and magnetic biochar prepared from chrysanthemum waste of the beverage industry, RSC Adv., 2023, 13(21), 14712–14728 , https://xlink.rsc.org/?DOI=D3RA01949G.
34. W. Duan, J. He, Z. Wei, Z. Dai and C. Feng, A unique Si-doped carbon nanocatalyst for peroxymonosulfate (PMS) activation: insights into the singlet oxygen generation mechanism and the abnormal salt effect, Environ. Sci.:Nano, 2020, 7(10), 2982–2994 , https://xlink.rsc.org/?DOI=D0EN00848F.
35. X. Liu, Z. Chen, S. Lu, X. Shi, F. Qu and D. Cheng, et al. Persistent free radicals on biochar for its catalytic capability: A review, Water Res., 2024, 250, 120999 , https://linkinghub.elsevier.com/retrieve/pii/S0043135423014392.
36. C. Dong, W. Fang, Q. Yi and J. Zhang, A comprehensive review on reactive oxygen species (ROS) in advanced oxidation processes (AOPs), Chemosphere, 2022, 308, 136205 , https://linkinghub.elsevier.com/retrieve/pii/S0045653522026984.
37. Y. Li, X. Zhao, Y. Yan, J. Yan, Y. Pan, Y. Zhang and B. Lai, Enhanced sulfamethoxazole degradation by peroxymonosulfate activation with sulfide-modified microscale zero-valent iron (S-mFe0): Performance, mechanisms, and the role of sulfur species, Chem. Eng. J., 2019, 376, 121302,  DOI:10.1016/j.cej.2019.03.178.
38. R. Bajagain and S. W. Jeong, Degradation of petroleum hydrocarbons in soil via advanced oxidation process using peroxymonosulfate activated by nanoscale zero-valent iron, Chemosphere, 2021, 270, 128627,  DOI:10.1016/j.chemosphere.2020.128627n.
39. J. Cao, L. Lai, B. Lai, G. Yao, X. Chen and L. Song, Degradation of tetracycline by peroxymonosulfate activated with zero-valent iron: performance, intermediates, toxicity and mechanism, Chem. Eng. J., 2019, 364, 45–56,  DOI:10.1016/j.cej.2019.01.113.
40. M. Voigt and M. Jaeger, Efficiency increased advanced oxidation processes by persalts for the elimination of pharmaceuticals in waterbodies: a short review, Discover Chem. Eng., 2024, 4(1), 16,  DOI:10.1007/s43938-024-00052-x.
41. Y. Gu, S. Sun, Y. Liu, M. Dong and Q. Yang, Solvent Effect on the Solvothermal Synthesis of Mesoporous NiO Catalysts for Activation of Peroxymonosulfate to Degrade Organic Dyes, ACS Omega, 2019, 4(18), 17672–17683,  DOI:10.1021/acsomega.9b01883.
42. J. Jing, X. Wang and M. Zhou, Electro-Enhanced Activation of Peroxymonosulfate by a Novel Perovskite-Doped Ti₄O₇ Anode with Ultra-High Efficiency and Low Energy Consumption: The Generation and Dominant Role of Singlet Oxygen, SSRN Electron. J., 2023, 232, 119682,  DOI:10.1016/j.watres.2023.119682.
43. A. K. Prasad and P. C. Mishra, Scavenging of superoxide radical anion and hydroxyl radical by urea, thiourea, selenourea and their derivatives without any catalyst: A theoretical study, Chem. Phys. Lett., 2017, 684, 197–204 , https://linkinghub.elsevier.com/retrieve/pii/S0009261417305808.
44. T. Chen, Z. Zhu, Y. Wang, H. Zhang, Y. Qiu and D. Yin, Efficient organics heterogeneous degradation by spinel CuFe₂O₄ supported porous carbon nitride catalyst: Multiple electron transfer pathways for reactive oxygen species generation, Chemosphere, 2022, 300, 134511 , https://linkinghub.elsevier.com/retrieve/pii/S0045653522010049.
45. P. A. Johnson, L. J. Bartolotti, P. W. Ayers, T. Fievez and P. Geerlings, Charge Density and Chemical Reactions: A Unified View from Conceptual DFT, in Modern Charge-Density Analysis, ed. C. Gatti and P. Macchi, Springer Netherlands, Dordrecht, 2011, pp. 715–764,  DOI:10.1007/978-90-481-3836-4_21.
46. American Public Health Association, Standard methods for the examination of water and wastewater, American Public Health Association, 1926.
47. I. M. Sayre, International Standards for Drinking Water, J. AWWA, 1988, 80(1), 53–60,  DOI:10.1002/j.1551-8833.1988.tb02980.x.
48. Y. Zhang, Z. Jiao, Y. Hu, S. Lv, H. Fan and Y. Zeng, et al. Removal of tetracycline and oxytetracycline from water by magnetic Fe₃O₄@graphene, Environ. Sci. Pollut. Res., 2017, 24(3), 2987–2995,  DOI:10.1007/s11356-016-7964-7.
49. C. Lai, D. Ma, H. Yi, M. Zhang, F. Xu and X. Huo, et al. Functional partition of Fe and Ti co-doped g-C3N4 for photo-Fenton degradation of oxytetracycline: Performance, mechanism, and DFT study, Sep. Purif. Technol., 2023, 306, 122546 , https://linkinghub.elsevier.com/retrieve/pii/S1383586622021025.
50. H. Jiang, Z. Qi and Z. Wang, Electrochemical-enhanced Fe₃O₄/biochar activates peroxymonosulfate (E/nano-Fe3O4/BC/PMS) for degradation of oxytetracycline, Chemosphere, 2022, 308, 136148 , https://linkinghub.elsevier.com/retrieve/pii/S0045653522026418.
51. Y. Fan, J. Su, Z. Wang, S. Liu, X. Li and S. Cao, Degradation of oxytetracycline in wastewater based on activated persulfate by biosynthesized iron oxide nanoparticles and carbon nanotube-modified biochar, J. Environ. Chem. Eng., 2023, 11(5), 110377 , https://linkinghub.elsevier.com/retrieve/pii/S2213343723011168.
52. X. Liu, Y. Pei, M. Cao, H. Yang and Y. Li, Magnetic CuFe₂O₄ nanoparticles anchored on N-doped carbon for activated peroxymonosulfate removal of oxytetracycline from water: Radical and non-radical pathways, Chemosphere, 2023, 334, 139025 , https://linkinghub.elsevier.com/retrieve/pii/S0045653523012924.
53. L. Jing, W. Yang, T. Wang, J. Wang, X. Kong and S. Lv, et al. Porous boron nitride micro-nanotubes efficiently anchor CoFe₂O₄ as a magnetic recyclable catalyst for peroxymonosulfate activation and oxytetracycline rapid degradation, Sep. Purif. Technol., 2022, 290, 120925 , https://linkinghub.elsevier.com/retrieve/pii/S1383586622004828.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ew00912f

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