Visible-light-driven Z-scheme tungsten oxide/ silver phosphate/ coffee bean biochar photocatalyst for simultaneous degradation of ciprofloxacin and chlorpyrifos

Abstract

The integration of biochar as a solid-state electron mediator in Z-scheme photocatalytic systems represents a promising approach for the efficient degradation of organic contaminants. In this study, a novel magnetic Z-scheme photocatalyst, silver phosphate–tungsten oxide (Ag₃PO₄–WO₃)/coffee bean biochar composite (ABW-10), was synthesized via a hydrothermal method. Ag₃PO₄ and WO₃ were immobilized onto biochar layers to facilitate charge separation and enhance electron transfer under visible light irradiation. Morphological analysis confirmed a uniformly decorated biochar surface with well-dispersed nanoparticles, supporting efficient photocatalytic performance. Mott–Schottky plots revealed improved charge carrier dynamics and conductivity. Radical scavenging experiments validated the Z-scheme charge transfer mechanism, confirming the generation of reactive oxygen species. ABW-10 exhibited exceptional photocatalytic activity, achieving nearly 100% mineralization of ciprofloxacin (50 ppm) and chlorpyrifos (20 ppm) within 60 minutes under ambient conditions at a catalyst dosage of 0.1 g. LC-MS analysis elucidated the degradation pathways of both pollutants. Additionally, the composite demonstrated excellent reusability and a fivefold increase in antimicrobial activity against both Staphylococcus aureus and Escherichia coli compared to the individual components.

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1. Introduction

The exponential growth in the global population, rapid urbanization, and accelerated industrialization have exerted unprecedented pressure on water resources. Human activities, including the discharge of industrial effluents, agricultural runoff, and untreated municipal waste, have turned vital water bodies into reservoirs of hazardous contaminants, severely compromising water quality and limiting access to clean and safe water.¹ As the demand for potable water continues to rise, innovative and sustainable strategies to restore and protect aquatic ecosystems are urgently required.^2,3 Among the diverse pollutants, antibiotics, pesticides, and pathogenic microorganisms pose significant environmental and health challenges. The widespread and indiscriminate use of antibiotics, such as ciprofloxacin, in human and veterinary medicine has led to their accumulation in aquatic environments due to poor metabolization rates. These compounds persist in wastewater and surface waters at concentrations ranging from ng L⁻¹ to μg L⁻¹, exhibiting toxic, mutagenic, and carcinogenic properties that conventional sewage treatment plants struggle to remove.^4–6 Photocatalytic degradation has emerged as a promising strategy for breaking down such persistent pharmaceutical pollutants.^7,8

Pesticide contamination, particularly from organophosphates like chlorpyrifos (CPF), further exacerbates environmental degradation. Despite its widespread agricultural use, only a small fraction (0.1%) of applied chlorpyrifos reaches target pests, with the remainder dispersing into the environment, leading to groundwater contamination and ecological disruption.^9,10 CPF's long environmental half-life (60–120 days) and neurotoxic effects, particularly on children and fetuses, necessitate effective remediation strategies.^11,12 Bioremediation using native and resistant microbial communities offers an eco-friendly and efficient solution for the restoration of pesticide-contaminated soils.^13,14 In parallel, pathogenic contamination by bacteria such as Escherichia coli and Staphylococcus aureus poses a serious public health risk, particularly in low-income countries, where it contributes to waterborne diseases and severe health complications.^15,16 The urgent need for economical and eco-friendly solutions to combat pathogenic contamination has gained renewed attention, especially in the wake of the global pandemic.

Advanced nanocomposite materials have emerged as transformative solutions for addressing these environmental challenges. Silver phosphate (Ag₃PO₄) is known for its high photocatalytic efficiency under visible light irradiation.¹⁷ However, its practical application is hindered by photodegradation, where photo-generated electrons reduce silver ions to metallic silver, leading to the deterioration of photocatalytic activity.¹⁸ Recent studies have demonstrated that coupling Ag₃PO₄ with other semiconductors can improve charge separation and stability.¹⁹ Among these, tungstate oxide (WO₃) stands out for its remarkable photocatalytic properties, exceptional stability, and light absorption capabilities in the visible and ultraviolet spectrum.²⁰

Incorporating biochar as a ESI† for Ag₃PO₄ and WO₃ offers a novel approach to enhancing photocatalytic performance, as supported by different studies demonstrating the effective coupling of photocatalysts with carbonaceous materials to improve charge separation and stability.^21,22 Biochar, derived from biomass pyrolysis, provides high surface area, porosity, and excellent adsorption capacity, facilitating the immobilization of nanoparticles and improving photocatalytic stability.²³ This synergistic system of Ag₃PO₄/WO₃/biochar nanocomposite demonstrates enhanced photocatalytic activity, efficient pollutant degradation, and easy recovery of the photocatalyst. Biochar's stable chemical properties and π-conjugated structure enable dynamic electron transfer, further enhancing the photocatalytic process.

This study explores the potential of the Ag₃PO₄/WO₃/biochar nanocomposite for visible light-driven degradation of aquatic pollutants, addressing critical environmental challenges and contributing to sustainable water treatment solutions.

2. Experimental section

2.1 Reagents

The primary raw material used in this research was waste coffee beans collected from the foothills of the Deomali range in the Koraput region of Odisha. Various high-purity chemicals were employed throughout the study to ensure precise synthesis and characterization processes. These included silver nitrate (AgNO₃), ciprofloxacin (CPX), sodium tungstate dihydrate (Na₂WO₄·2H₂O), sodium sulfate (Na₂SO₄), chlorpyrifos (CPF), benzoquinone (BQ), isopropanol (IPA), potassium dichromate (K₂Cr₂O₇), and disodium ethylenediaminetetraacetate (Na₂EDTA), all sourced from Sigma Aldrich with an assay purity of 99.0%. Additionally, ethanol and concentrated hydrochloric acid (HCl), also with 99.0% assay purity, were procured from Himedia. Double-distilled water was meticulously used to prepare all aqueous solutions for the synthesis process, ensuring high-quality results in the development of hybrid nanocomposites.

2.2. Synthesis of biochar

Meticulous procedures were followed to prepare the coffee bean biomass for subsequent examination and applications. To ensure the integrity of the biomass, it was initially washed with tap water and then thoroughly rinsed with distilled water to remove surface impurities. The cleaned biomass was carefully sectioned into smaller pieces to maximize surface area for further treatments. Moisture content was effectively eliminated by drying the material in an oven at 100 °C until a constant weight was achieved, a critical step for subsequent processing. The dried biomass was finely ground using a pulveriser fitted with a 250 mm mesh, ensuring uniformity and enhanced reactivity. The key synthesis step, pyrolysis, was conducted in a muffle furnace at 500 °C for one hour under a continuous nitrogen gas flow, with a controlled heating rate of 10 °C min⁻¹. This controlled thermal decomposition process was essential in transforming the biomass into a functional material, establishing a foundation for its potential applications in diverse environmental and industrial fields.

2.3. Synthesis of silver phosphate (Ag₃PO₄)

For the synthesis of silver phosphate, solutions of AgNO₃ (0.3 M, 1.27 g) and (NH₄)₂H₂PO₄ (0.45 M, 1.29 g) were prepared separately in beakers. The (NH₄)₂H₂PO₄ solution was carefully added to the AgNO₃ solution over four hours at 50 °C with continuous stirring, resulting in the formation of a distinct yellow precipitate. The silver phosphate obtained was thoroughly washed with a 75/25 (v/v%) ethanol–water solution to remove any residual impurities. This vigorous cleaning process ensured the purity of the synthesized product. The washed material was then dried at 70 °C overnight, yielding clean and stable silver phosphate for further characterization and applications.

2.4. Synthesis of tungstate oxide (WO₃)

The synthesis of tungstate oxide (WO₃) was accomplished through a multistep process. Initially, 2.5 grams of sodium tungstate dihydrate was dissolved in 40 mL of deionized water under mild stirring to form a homogeneous solution. To this solution, 4 grams of sodium sulfate (Na₂SO₄) were added, ensuring uniform mixing. The pH of the suspension was meticulously adjusted to 1.5 by the gradual addition of concentrated hydrochloric acid (HCl). Following pH regulation, the solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave, which was sealed and subjected to hydrothermal treatment at 180 °C for 5 hours. After cooling to room temperature, the sample was carefully extracted and thoroughly washed with ethanol and deionized water to eliminate any residual impurities. The purified product was then dried overnight at 80 °C, yielding high-purity tungstate oxide (WO₃) for further applications.

2.5. Synthesis of Z-scheme nanocomposite (Ag₃PO₄/WO₃/biochar) ABW-10

A two-step synthesis method was employed to prepare tungsten oxide (WO₃)/silver phosphate (Ag₃PO₄)/biochar hybrid composites with a stoichiometric biochar/Ag₃PO₄ content of 10 wt%. In the initial step, WO₃ (80 wt%) and biochar (10 wt%) were dispersed in 20 mL of distilled water and homogenized using ultrasonic vibration for 30 minutes. Concurrently, a solution containing silver nitrate (0.3 M) and (NH₄)₂H₂PO₄ (0.45 M) was prepared in a three-necked round-bottom flask. The ultrasonicated WO₃/biochar dispersion was gradually added to the silver phosphate solution under continuous stirring for one hour. The mixture was then heated at 50 °C with constant stirring for 3 hours, resulting in the formation of a black solid comprising Ag₃PO₄/WO₃/biochar, maintaining a 10 wt% biochar/Ag₃PO₄ ratio. The solid product was recovered by vacuum filtration, washed thoroughly with a 70/30% (v/v) ethanol–water solution, and dried at 80 °C overnight. The resulting composite, designated as ABW-10, was subsequently evaluated for its organic pollutant mineralization efficiency. The schematic representation of the synthesis process is illustrated in Fig. 1.

Fig. 1 Schematic presentation of the synthesis process of (a) tungstate oxide, (b) silver phosphate, and (c) ABW-10 composite.

2.6. Characterisation

X-ray diffraction (XRD) was used to examine the crystallinity and multiphase composition of all produced samples (PANalytical X'Pert powder diffractometer). The measurements were carried out using Cu Kα radiation with a wavelength of 1.5406 Å. The important bonds in the samples were identified using an FTIR spectrophotometer (IRAffinity-1S, Shimadzu). The optical absorption properties of the synthesized samples were investigated using UV–vis diffuse reflectance spectroscopy (Shimadzu UV-2400) equipped with an integrating sphere attachment. The measurements were carried out in the wavelength range of 200–800 nm using BaSO₄ as the reflectance standard. These absorbance values, plotted against wavelength, were then used to estimate the band gap energies using Tauc's plots derived from the Kubelka–Munk function, which is the standard method for band gap estimation from UV-Visible diffuse reflectance spectra (DRS). The photoluminescence (PL) spectra were scanned on an RF-6000 spectrofluorometer to investigate the photogenerated charge carrier recombination rate. A scanning electron microscope (SEM, Zeiss EVO) and a high-resolution transmission electron microscope (HRTEM, Jeol Jem F200) with a selected area electron diffraction pattern (SAED) were used for the morphological investigations. Energy dispersive X-ray spectroscopy (EDX) and elemental mapping revealed the presence of elements (Ag, W, O, C, N, and P) and their proportions. Lastly, liquid chromatography-mass spectrometry (LC-MS) measurements on a Shimadzu LC-MS/MS 8045 device with electrospray ionization mass spectrometry were used to examine the photocatalytic degradation products. This analysis revealed information about the composition and transformation of the degradation products.

3. Result and discussion

3.1. Morphological analysis

The morphology and interfacial characteristics of as-prepared photocatalyst materials, including Ag₃PO₄, WO₃, biochar, and ABW-10, were investigated using scanning electron microscopy (SEM) as shown in Fig. 2a–e. The SEM image of pure Ag₃PO₄ (Fig. 2b) reveals the presence of agglomerated quasi-spherical nanoparticles with sizes generally below 100 nm. WO₃ (Fig. 2c) exhibits rod-like and spherical morphologies ranging from ∼50–100 nm. The composite ABW-10 (Fig. 2d) shows a uniformly distributed heterostructure, indicating successful integration of all components at the nanoscale.

Fig. 2 SEM image of the (a) biochar, (b) Ag₃PO₄, (c) WO₃, (d) ABW-10, and (e) EDS of ABW-10.

This integration indicated successful synthesis of the Z-scheme ABW-10 system via a facile hydrothermal process, with structures grown on continuous layers of biochar. However, intergranular nanoparticle aggregation was observed in the ABW-10 nanocomposite, potentially due to interfacial grain reactivity influenced by exchanging dipole moments of particles with wide surface-active sites in the system.

Transmission electron microscopy (TEM) analysis was employed to gain deeper insights into the interfacial connection between WO₃ and Ag₃PO₄ particles on the biochar layers within the ABW-10 photocatalytic system. Fig. 3a illustrates the close integration of WO₃ and Ag₃PO₄ particles with the biochar sheet. The acquired TEM images reveal the multi-layered structure of biochar, exposing numerous edges conducive to catalytic reactions. Additionally, the images demonstrate the well-ornamented biochar layers, with large WO₃ particles and tiny Ag₃PO₄ particles uniformly distributed across the surface (Fig. 3b). Unlike scanning electron microscopy (SEM), TEM analysis allows for the clear visualization of individual semiconductor catalysts, eliminating the issue of particle aggregation observed in SEM data. The precise particle sizes of individual catalysts were thus computable. Specifically, TEM analysis revealed that WO₃ particles are in direct contact with biochar layers, with sizes ranging from 100 to 300 nm. In contrast, Ag₃PO₄ nanoparticles were observed to be smaller, ranging from 10 to 30 nm, and evenly dispersed over the biochar layers.

Fig. 3 TEM analysis of (a) close integration of WO_3, Ag₃PO_4, and biochar in ABW-10, (b) uniform distribution of particles in ABW-10 composite.

3.2. Structural analysis

The data revealed from the HRTEM survey was found to be more compatible with the XRD data analysis, as could be found in Fig. 4. It was found that the XRD pattern of WO₃ nanoparticles depicts different peaks at 2θ = 14.0°, 22.78°, 28.21°, 37.62°, 49.94°, 55° of the hexagonal structure that are well matched to the (100), (001), (200), (210), (220) and (202)planes, respectively (JCPDS standard data, card no. 01-075-2187).²⁴ Regarding biochar, the XRD pattern displays a distinct peak at 2θ = 30° because of the plane of (002). Due to the small amount of biochar present in the ABW-10 composite in the XRD examination, this peak was not apparent and was discovered in the ABW-10. Ag₃PO₄'s powder diffraction pattern (JCPDS code number 06-0505) indicates that the crystal shape is cubic. Similar peak patterns can be seen in the diffraction patterns of the ABW-10 composites, including those of the Ag₃PO₄ and WO₃ peaks, Ca. (210), (220), (200), and (320), among others. This demonstrates that the ABW-10 composite was successfully formed via the hydrothermal technique. Fig. 4c and d shows the results of an additional inspection of the WO₃ and Ag₃PO₄ microscopic structures using high-resolution TEM (HRTEM) images obtained from SAED. As seen in the figure, the diffraction ring pattern consists of a bright and dark pattern grouped in a ring-like configuration. The discontinuous ring patterns, or spotty rings, are visible in the SAED photos of the ABW-10 samples. The values in the standard data (JCPDS card no. 01-075-2187) and (JCPDS card no. 06-0505) are in good agreement with the measured inter-planar spacing (d_hkl) from SAED patterns (Fig. 4c). The (310), (211), (200), (320), (220), and (210) planes are recognized as the diffraction rings, arranged from inner to outer, and they clearly show the presence of both semiconductor crystals. Once more, we examined the spacings between neighbouring lattice fringes and found that they were 0.28 nm and 0.27 nm, respectively. These values matched the d-spacings of the (220) WO₃ and (210) Ag₃PO₄ nanoparticle planes, indicating a high degree of particle connectivity on the polymer surface.²⁶ We can identify the heterostructure nanocomposite's polycrystallinity (Fig. 4d) using the ABW-10 composite's SAED array. The concentric ring pattern of the ABW-10 composite is attributed to the planes of the two semiconductors on the biochar layer surface.^25,26

Fig. 4 XRD of (a) biochar, and (b) WO₃, Ag₃PO₄, and ABW-10 catalyst, (c) corresponding HRTEM analysis and SAED pattern of ABW-10, and (d) polycrystallinity nature of ABW-10 composite.

3.3. Optical analysis and band gap determination

The optical absorption characteristics of WO₃, Ag₃PO₄, and the ternary composite ABW-10 were systematically examined using UV–vis diffuse reflectance spectroscopy (DRS), as illustrated in Fig. 5a–c. The results indicate that WO₃ and Ag₃PO₄ exhibit significant absorption within the visible light spectrum, which is further enhanced upon the incorporation of biochar and Ag₃PO₄ onto the WO₃ surface. The resulting ABW-10 heterostructure photocatalyst demonstrated a broader absorption range spanning the entire solar spectrum (200–800 nm), surpassing the individual components. This enhancement in optical absorption, particularly in the visible region, suggests improved light-harvesting efficiency, which is critical for photocatalytic applications. Furthermore, photoluminescence (PL) spectroscopy (Fig. 4h) was employed to evaluate charge carrier behavior under photon excitation at λ_ex = 420 nm. The observed reduction in PL intensity upon the incorporation of biochar and Ag₃PO₄ indicates suppressed radiative recombination of photogenerated electron–hole pairs. The lowest PL intensity recorded for ABW-10 indicates enhanced charge separation and suppressed electron–hole recombination, which contributes to its improved photocatalytic performance.

Fig. 5 UV-vis DRS spectra and inset Tauc plots of (a) WO₃, (b) Ag₃PO₄ and (c) ABW-10, FTIR spectra of (d) Ag₃PO₄, (e) WO₃, (f) coffee bean biochar and (g) ABW-10 nanocomposite and (h) PL spectra of biochar, Ag₃PO₄, WO₃ and ABW-10.

The structural and functional group analysis of the synthesized catalysts was further investigated using Fourier transform infrared (FTIR) spectroscopy, as presented in Fig. 5d–g. The FTIR spectrum of ABW-10 exhibits distinct absorption bands at approximately 3444 cm⁻¹, 2361 cm⁻¹, 1656 cm⁻¹, and 866 cm⁻¹, corresponding to O–H stretching, C≡C stretching, PO₄³⁻ stretching, and P–O–P stretching vibrations, respectively, confirming the presence of characteristic functional groups within the composite. Additionally, a broad peak at 817 cm⁻¹ was attributed to W–O–W stretching vibrations, further validating the incorporation of WO₃. In the 900–1100 cm⁻¹ region, a narrow broad band was identified, consistent with the vibrational features of WO₃.¹⁸ The FTIR spectrum of pristine Ag₃PO₄ displayed characteristic peaks at 3353 cm⁻¹ and 866 cm⁻¹, corresponding to asymmetric O–H stretching and O=P–O bending vibrations, respectively. For the coffee bean biochar, characteristic absorption peaks at 784 cm⁻¹, 1670 cm⁻¹, 2361 cm⁻¹, and 3430 cm⁻¹ were assigned to C–H stretching, C=O stretching, C≡C stretching, and O–H stretching vibrations.¹⁷ The presence of these functional groups and bonding interactions in the ABW-10 composite confirms the successful formation of the heterostructure catalyst, demonstrating its structural integrity and suitability for enhanced photocatalytic applications.

The band gap energy (E_g) of a semiconductor can be determined by extrapolating the linear portion of the (αhν)ⁿ versus hν, as depicted in Fig. 5a–c. The calculated band gap values for Ag₃PO₄, WO₃, and the ABW-10 composite were found to be 2.4 eV, 2.85 eV, and 2.1 eV, respectively. The band gap values of WO₃ and Ag₃PO₄ align well with previously reported data, affirming the reliability of the measurements. Notably, the incorporation of biochar in ABW-10 resulted in a significant reduction in the band gap energy, with a decrease of 0.3 eV compared to Ag₃PO₄ and 0.75 eV compared to WO₃. This band gap narrowing suggests enhanced electronic interactions within the heterostructure, primarily due to biochar's role as an electron shuttle, facilitating charge carrier mobility. The morphological analysis further reveals that the uniform dispersion of Ag₃PO₄ and WO₃ nanoparticles on biochar sheets forms conductive pathways throughout the composite matrix, thereby improving charge separation and transport. Furthermore, the unique surface properties of biochar contribute to the reduction in band gap energy by promoting an efficient electrical connection between WO₃ and Ag₃PO₄ within the Z-Scheme system.

The surface chemical states of the ABW-10 composite were further analysed using X-ray photoelectron spectroscopy (XPS), with all binding energies calibrated to the C 1s peak at 285 eV. The XPS spectrum of WO₃ nanorods, as shown in Fig. 6c, exhibits characteristic peaks at 35.0 eV and 36.7 eV, which correspond to the W 4f_7/2 and W 4f_5/2 orbitals of W⁶⁺, respectively. Additionally, a peak at 530.5 eV is attributed to the lattice oxygen in the WO₃ nanorods, Fig. 6d, further confirming the oxidation state of tungsten.²⁷ The XPS spectrum of silver (Fig. 6a) displays distinct peaks at 369 eV and 375 eV, corresponding to the Ag 3d_5/2 and Ag 3d_3/2 binding energies, respectively, confirming the presence of Ag⁺ ions in the composite. The absence of any detectable peaks related to metallic Ag⁰ indicates that no reduction to elemental silver occurred during the synthesis, which aligns well with X-ray diffraction (XRD) results. Furthermore, the P 2p scan (Fig. 6b) shows a binding energy peak at 133.5 eV, corresponding to the PO₄³⁻ species, verifying the incorporation of phosphate groups within the composite structure.²⁸ These results confirm the successful formation of the ABW-10 hybrid composite, demonstrating the effective integration of WO₃, Ag₃PO₄, and biochar in the synthesized material.

Fig. 6 XPS spectra of ABW-10 composite having (a) Ag 3d, (b) P 2p, (c) W 4f, and (d) O 1s, and (e) C 1s.

3.4. Contaminant removal analysis

The photocatalytic activity of various catalysts, including Ag₃PO₄, WO₃, biochar, and the synthesized ABW-10 composite, was systematically investigated for the degradation of the pesticide chlorpyrifos (CPF) and the antibiotic ciprofloxacin (CPX) under visible-light irradiation. Prior to light exposure, all samples were allowed to reach adsorption–desorption equilibrium in dark conditions. The photocatalytic efficiency of both individual catalysts and the ABW-10 composite was evaluated, as depicted in Fig. 7a and b. Comparative analysis revealed that ABW-10 exhibited superior photo-mineralization efficacy compared to its neat counterparts, highlighting its enhanced catalytic properties. It was found from the experimental analysis that the photodegradation of CPF without using photocatalyst ABW-10 is negligible less than 10% as shown in Fig. 7c.

Fig. 7 Comparative photodegradation analysis of (a) ciprofloxacin, and (b) chlorpyrifos using biochar, WO₃, Ag₃PO_4, and ABW-10 composite.

Further investigations into the degradation efficiency were conducted using varying concentrations of CPF and CPX (10–100 ppm) over different time intervals (Fig. 8c and d). These experiments demonstrated that complete photodegradation of a 50 mg L⁻¹ CPF and CPX solution was achieved within 60 minutes using an ABW-10 catalyst dose of 100 mg. However, an increase in CPF and CPX concentration led to a reduction in degradation efficiency, likely due to molecular aggregation at the active catalytic sites. This aggregation may hinder light penetration and interfere with the separation of photogenerated electron–hole (e⁻/h⁺) pairs, thereby reducing the overall photocatalytic degradation rate.

Fig. 8 (a and b) Photocatalytic activity of ABW-10 composites (catalyst dosage: 0.1 g) for (a) ciprofloxacin (50 mg L⁻¹), (b) chlorpyrifos (50 mg L⁻¹) pollutants degradation under visible light irradiation. (c and d) Photocatalytic activity of the ABW-10 composite with different concentrations of CPF and CPX, (e and f) Linear fitting of the visible light degradation profiles to the first-order kinetic model of CPF (30 mg L⁻¹) and CPX (20 mg L⁻¹) in the presence of the ABW-10 catalyst, (g) adsorption of CPF (20 mg L⁻¹) and CPX (20 mg L⁻¹) using biochar, WO_3, Ag₃PO₄, and ABW-10.

The UV-visible spectral scans (Fig. 8a and b) illustrate the temporal evolution of CPX and CPF concentrations during the degradation process. Notably, rapid degradation kinetics were observed (Fig. 8e and f), with CPX achieving nearly complete degradation within 60 minutes of irradiation, and CPF following a similar trend. The absence of any noticeable shift in the absorbance band throughout the degradation process indicates that CPF and CPX were mineralized into smaller, environmentally benign products such as CO₂ and ethene rather than merely breaking down into smaller organic fragments. Moreover, a recyclability test conducted over five successive cycles using a 50 mg L⁻¹ CPX solution demonstrated only a marginal decrease in photocatalytic activity, underscoring the stability and reusability of ABW-10.

The degradation kinetics followed a first-order reaction model, as confirmed by high correlation coefficient (R²) values. The degradation kinetics were plotted as ln(C₀/C_t) versus time (t) based on eqn (1),

ln C₀/C_t = kt (1)

where ‘k’ represents the rate constant, and ‘C₀’ and ‘C_t’ denote the initial and final concentrations of CPF or CPX at time t, respectively. This strong adherence to the first-order kinetic model further validates the efficiency of ABW-10 in the photocatalytic mineralization of CPX and CPF. In contrast to individual catalysts, the ABW-10 composite exhibited markedly enhanced reusability characteristics. The inherent susceptibility of pure Ag₃PO₄ to surface corrosion, resulting in degradation rates of 60–80% for CPX (at 50 ppm) and limited recycling potential, posed a significant challenge. Consequently, numerous investigations were undertaken to mitigate this self-corrosion propensity through surface modifications and doping strategies, including cobalt ferrite, PmAP doping, and facet engineering of Ag₃PO₄.²⁶ This study effectively addressed the corrosion inhibition of the Ag₃PO₄ structure within the ABW-10, consequently augmenting the recycling capability of the catalyst. In order to evaluate the adsorption efficiency of the individual components and the ABW-10 composite, batch adsorption experiments were conducted using CPF and CPX as target pollutants. Each adsorbent (biochar, WO₃, Ag₃PO₄, and ABW-10) was tested at a dosage of 0.1 g in a 100 mL solution containing 20 mg L⁻¹ of CPF or CPX under dark conditions for 60 minutes to ensure adsorption–desorption equilibrium. As illustrated in Fig. 8g, the adsorption capacities of the materials varied significantly. Biochar exhibited moderate adsorption with 15.8 mg g⁻¹ for CPF and 13.4 mg g⁻¹ for CPX, attributed to its porous structure and abundant surface functional groups. WO₃ and Ag₃PO₄ demonstrated comparatively lower adsorption capacities, indicating limited surface interactions with the pollutants. Notably, the ABW-10 composite showed the highest adsorption efficiency, with 19.3 mg g⁻¹ for CPF and 16.7 mg g⁻¹ for CPX.²⁸

3.5. LC-MS analysis

This study comprehensively investigated the photocatalytic degradation pathways of chlorpyrifos (CPF) and ciprofloxacin (CPX) using LC-MS analysis, which enabled the identification of intermediate and final degradation products. The LC-MS spectra revealed distinct mass signals for CPF (m/z 351, 170, 197, 239, 177, 114, 97, and 28) and CPX (m/z 331, 259, 229, 265, 212, 58, and 44), indicating the presence of both parent compounds and their degradation intermediates as shown in Fig. 1S and 2S (ESI†) respectively. Scavenging studies confirmed that hydroxyl radicals (˙OH) and photogenerated holes (h⁺) were the primary active species responsible for breaking down CPF and CPX into smaller by-products. The photocatalytic mechanism involved the excitation of CPF and CPX molecules, leading to electron transfer into the conduction band of ABW-10 and the subsequent formation of cationic radicals. These radicals interacted with oxygen species, producing oxidized intermediates that further degraded into aldehydes and carboxylic acids before complete mineralization into CO₂. The LC-MS spectra at the end of the reaction displayed molecular ion signals at lower m/z values, corresponding to CO₂, ethanol, ethene, and other minor organic species, confirming the effective degradation and mineralization process.

3.6. Photocatalytic mechanism

The Z-scheme photocatalytic mechanism of ABW-10 involving WO₃, Ag₃PO₄, and biochar under visible light irradiation presents an efficient pathway for enhanced degradation of pollutants such as chlorpyrifos (CPF) and ciprofloxacin (CPX). Mott–Schottky analysis, carried out using Staircase Potential Electrochemical Impedance Spectroscopy (SPEIS) in a 1 M Na₂SO₄ electrolyte, revealed that WO₃ is an n-type semiconductor with a conduction band (CB) at −0.17 eV and a band gap of 2.85 eV, placing its valence band (VB) at +2.68 eV. Conversely, Ag₃PO₄, a p-type semiconductor, shows a CB at −1.65 eV with a band gap of 2.4 eV, yielding a VB at +0.75 as shown in Fig. 9a and b respectively.

Fig. 9 (a) Mott–Schottky of WO₃ and (b) Ag₃PO₄, (c) A plausible mechanism of Z-scheme system e⁻/h⁺ pair transfer in ABW-10 under UV-Vis illumination of light, and (d) scavengers trapping experiments for photodegradation of ciprofloxacin by ABW-10.

Eqn (2) was utilized to determine the conversion between Ag/AgCl and RHE, with all potentials being referenced against the reversible hydrogen electrode (RHE).

E_RHE = E_Ag/AgCl + 0.059pH + 0.197 V (2)

Upon visible light illumination, both WO₃ and Ag₃PO₄ generate photoinduced electron–hole pairs. In the proposed direct Z-scheme system, electrons in the CB of WO₃ recombine with holes in the VB of Ag₃PO₄ through biochar, which acts as a solid-state electron mediator. This preserves the strong redox potentials of the remaining charge carrier electrons in the CB of Ag₃PO₄ (−1.65 eV) and holes in the VB of WO₃ (+2.68 eV) as shown in Fig. 9. The valence band potential and the conduction band were computed using eqn (3).

E_VB = E_g + E_CB (3)

Biochar's pivotal role stems from its partially graphitized structure, which enables high electrical conductivity and π-conjugation for rapid electron transfer. Its surface functional groups (e.g., –OH, –COOH) aid in temporary electron capture and shuttling, reducing recombination. Furthermore, biochar's porous architecture and large surface area enhance contact between WO₃ and Ag₃PO₄, facilitating charge transfer across the interface. Its inherent redox-capacitive behaviour allows it to store and release electrons effectively, supporting stable charge carrier dynamics even under fluctuating light conditions.

The active species trapping experiments further validate the mechanism. When 1 mmol L⁻¹ of scavengers were added benzoquinone (BQ) for ˙O₂⁻, isopropanol (IPA) for ˙OH, and EDTA–Na₂ for h⁺, the results showed a significant suppression in degradation efficiency upon BQ addition, confirming the major role of superoxide radicals (˙O₂⁻). IPA addition resulted in a ∼35% reduction in CPX degradation, implicating ˙OH radicals in the degradation process, while EDTA–Na₂ had minimal effect, indicating a lesser contribution from holes, as shown in Fig. 9d. In this Z-scheme configuration, the retained electrons in the CB of Ag₃PO₄ reduce adsorbed O₂ to generate ˙O₂⁻ radicals, while the holes in the VB of WO₃ directly oxidize pollutants such as CPF and CPX through the hydroxyl radical (˙OH). This synergistic charge separation, facilitated by biochar, enables efficient utilization of reactive oxygen species and strong oxidative holes, thereby enhancing the photocatalytic degradation of environmental contaminants.

3.7. Antimicrobial study

The antibacterial activity of ABW-10, WO₃, Ag₃PO₄, and biochar was systematically evaluated against E. coli and S. aureus using the agar well diffusion assay, revealing a concentration-dependent inhibitory effect. Among the tested materials, ABW-10 nanoparticles exhibited the highest antibacterial efficacy, with a zone of inhibition (ZOI) of 22 mm against E. coli and 19 mm against S. aureus at the maximum concentration of 1000 mg mL⁻¹. As illustrated in Fig. 10a–d, from Fig. 10e, the ZOI for individual components against E. coli was recorded as 3 mm for biochar, 11 mm for Ag₃PO₄, and 10 mm for WO₃, while for S. aureus, the ZOI values were 2 mm, 9 mm, and 8 mm, respectively. These results underscore the superior antibacterial performance of ABW-10, likely due to the synergistic effect of its composite constituents. A comparative ZOI against Gram-positive and Gram-negative bacteria has been shown in Table 1. Furthermore, the comparative analysis presented in Table 1 highlights the relative antibacterial effectiveness of these photocatalysts, reinforcing the potential of ABW-10 as a promising antimicrobial agent.

Fig. 10 Zone of inhibition at concentration of 1000 μg mL⁻¹ in Escherichia coli (E. coli) with ABW-10 (a), WO₃ (b) and in S. aureus (S. aureus) WO₃ (c) and ABW-10 (d) and comparative study of individual catalyst ZOI at concentration of 1000 μg mL⁻¹ using Gram negative (E. coli) and Gram positive (S. aureus).

Table 1 Comparative study of zone of inhibition (ZOI) for different semiconductors

Hybrid material	ZOI		References
	S. aureus	E. coli
Copper oxide nanoparticles	14.5	9.33	29
Chitosan–silver nanoparticles (CS–AgNPs)	—	10	30
Methanol silver nanoparticles (SNPs)	14.33	10.33	31
AgNP-embedded guar gum/gelatin nanocomposite	13	12.5	32
Copper–silver–titanium oxide nanocomposite (Cu–Ag–TiO2)	15	19	33
Magnesium/zinc oxide	19	15	34
ABW-10	19	22	This work

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3.8. Reusability analysis

The stability and recyclability of the catalyst used in photocatalytic processes are critical factors that determine their economic viability. In this case, five consecutive cycles of the same experimental setup were used to thoroughly assess the stability and recyclability of the ABW-10 composite. The results, as shown in Fig. 11a and b, indicate that the composite's degradation percentage remained relatively constant even after five cycles, indicating its strong durability during the CPX and CPF degradation process. Moreover, an investigation using powder X-ray diffraction (PXRD) after the fifth cycle, as shown in Fig. 11c, which confirms the ABW-10 catalyst's continued stability.

Fig. 11 Recyclability of photodegradation of (a) ciprofloxacin, and (b) chlorpyrifos of ABW-10 composite after the 5th cycle.

4. Conclusion

This study demonstrates the promising potential of utilizing biochar as an electron mediator in a Z-scheme system photocatalytic system, particularly in conjunction with WO₃ and Ag₃PO₄. The synthesized WO₃/Ag₃PO₄/biochar heterostructure, notably the ABW-10 catalyst, exhibited exceptional performance in the mineralization of organic contaminants under visible light irradiation. Through comprehensive characterization techniques, including PL spectroscopy, XRD, FTIR, and SEM/TEM morphological analysis, it was confirmed that the ABW-10 heterostructure possessed favourable properties such as reduced charge carrier recombination, abundant active sites, and intimate interfaces between components, facilitating rapid electron–hole pair transmission and extended charge carrier lifetime. The enhanced photocatalytic activity of the ABW-10 heterostructure, leading to complete degradation of the antioxidant ciprofloxacin pollutant and noxious chlorpyrifos in 60 minutes, surpasses that of individual WO₃ and Ag₃PO₄ catalysts. This enhanced performance can be attributed to the synergistic effects and strong interaction among the components within the composite heterostructure. Moreover, the stability of the heterostructure Z-scheme system photocatalysts under visible light irradiation further underscores their potential for practical applications in the treatment of antibiotic and pesticide containing wastewater and other environmental pollutant cleanup processes. In summary, the ABW-10 catalyst exhibits favourable characteristics such as proper Z-scheme system band alignment, efficient electron–hole migration, and high light absorption capacity, making it a promising candidate for advancing photocatalysis-based solutions for energy and environmental waste remediation. Continued research in this area holds great promise for addressing pressing environmental challenges and advancing sustainable technology solutions.

Consent to participate and publish

All the authors consented to participate in drafting and publishing this manuscript.

Data availability

The data that support the findings of this study are available from the corresponding author [Behera, A. K.] upon reasonable request.

Author contributions

Debasis Nanda: Writing – original draft, data curation, and visualization; Chirasmayee Mohanty: writing and editing the original draft, visualization, conceptualization; Amrita Priyadarsini: software, formal analysis, data curation, methodology; Nigamananda Das: software, formal analysis, data curation; Manjusri Misra: Software, formal analysis, data curation, methodology; Amar K. Mohanty: data curation and formal analysis; Ajaya K. Behera: methodology, supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Debasis Nanda and Chirasmayee Mohanty have contributed equally. We are grateful to the Department of Chemistry, Utkal University, Bhubaneswar for allowing the smooth conduct of experiments. We are also grateful to Central Instrumentation Facility, Odisha University of Technological Research, Bhubaneswar for conducting scanning electron microscopy for the samples.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ra04526f

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