Photocatalytic degradation and dechlorination mechanism of diclofenac using heterojunction Mn-doped tungsten trioxide (Mn-WO3) nanoparticles under LED visible light from aqueous solutions (2024)

Characterization of Mn-WO₃ nanocomposite

UV-Vis DRS was employed to investigate the band gap energy and reflectance spectroscopy of WO₃ and Mn-WO₃ nanocomposites, as shown in Fig.2a and b. The obtained results revealed the following observations: Pure WO₃ particles exhibited visible light absorption within the UV wavelength range of 200 to 475nm. Figure2a displays the spectra of the Mn-WO₃ nanocomposite. It was observed that with an increase in Mn content within the nanocomposite structure, the absorption edge experienced a noticeable change. This alteration in absorption edge can be attributed to the influence of Mn doping on the electronic structure and optical properties of the WO₃ host material. These findings indicate that the incorporation of Mn into the WO₃ lattice has a significant impact on the light absorption characteristics of the resulting nanocomposite, potentially affecting its photocatalytic performance under visible light irradiation. Further analysis is necessary to understand the exact role of Mn in enhancing the photocatalytic activity of the Mn-WO₃ nanocomposite.

It can be concluded that the incorporation of Mn at various contents (20, 15, and 5 wt%) into WO3 enhanced the photoactivation of the resulting Mn-WO₃ nanocomposite at higher wavelengths. Notably, the 15 wt% Mn-doped WO₃ nanocomposite exhibited a wide absorption band ranging from 200 to 540nm, indicating an increase in visible light absorption. To determine the band gap energy of the Mn-WO₃ nanocomposite, the Kubelka-Munk model can be applied. The band gap of the host material was found to be altered upon doping with Mn. The band gap can be calculated using Tauc’s theory, which relates the energy-dependent absorption coefficient to the band gap energy (Eg) through the following equation:

where h is the Planck constant, ν is the photon’s frequency, Eg is the band gap energy, and B is a constant. The γ factor depends on the nature of the electron transition and is equal to 1/2 or 2 for the direct and indirect transition band gaps, respectively. If we consider Eq.6 with the following parameters, the equation will be a line.

where this line intersects the graph will be equal to the band gap energy.

That’s mean: \(Y = B\left( {X - Eg} \right),\,\,if~Y = 0 \to B\left( {X - Eg} \right) = 0 \to X = Eg\)

To find the bandgap, a line with a positive slope must fit the curved part of the graph. The point where this line intersects the horizontal axis will be equal to the band gap energy. For powder samples, the reflectance spectrum is recorded from DR S spectrophotometry of the sample, and the Tauc equation for the reflectance spectrum will be as follows³⁸.

The F function is calculated from the Kubelka-Munk equation as follows from the reflection coefficient.

where R is the reflectance of an infinitely thick specimen.

As illustrated in Fig.2b, the band gap energies of WO₃ with 5, 15, and 20 wt% Mn doping, as well as pure WO₃, were determined to be 2.66, 2.5, 2.62, and 2.75eV, respectively. These results indicate that Mn doping effectively decreased the band gap energy of WO₃. Consequently, the 15 wt% Mn-doped WO₃ (Mn-WO₃) nanocomposite exhibited higher light absorption intensity with a lower optical band gap (2.5eV) and demonstrated enhanced activation under LED visible light irradiation. The decrease in band gap is likely due to the creation of an intermediate energy level between the conduction and valance bands of WO3 NPs²⁹. The increased light absorption leads to a higher generation of photo-induced holes and electrons, ultimately resulting in enhanced generation of hydroxyl radicals and decomposition efficiency²⁰ The separation efficiency of photo-induced electron-hole pairs and photocurrent intensity were evaluated using transient photocurrent responses of WO₃, Mn-WO₃ (5%), Mn-WO₃ (15%), and Mn-WO₃ (20%) (Fig.2c). As shown in Fig.2c, pure WO₃ NPsand Mn-WO₃ nanocomposite samples generated stable photocurrent responses under visible light irradiation, which rapidly decreased when the light was turned off. The Mn(15% wt)-WO₃nanocomposite demonstrated stronger photocurrent intensity over multiple light on/off cycles, indicating that doping increases the charge carriers’ dissociation efficacy in the photocatalyst. To investigate the electrodes’ conductive performance, charge transfer, and separation of electron-hole pairs, electrochemical impedance spectroscopy (EIS) was employed to evaluate the electrolyte dispersion in the electrode/electrolyte junction (Fig.2d). The EIS Nyquist plots of Mn-WO₃ (5%), Mn-WO₃ (15%), Mn-WO₃ (20%), and WO₃showed that an increase in the separation efficiency and transfer of charge carriers led to a decrease in the arc radius²⁹. In the EIS spectra (Fig.2d), the Mn(15% wt)-WO₃ nanocomposite displayed a significantly smaller semicircle compared to other samples, suggesting enhanced charge transfer ability at the electrode/electrolyte interface and improved electrode conductivity. The incorporation of Mn in WO₃NPs substantially enhanced the transfer and separation of charge carriers, resulting in more accessible active species and increased photocatalytic activity of the Mn-WO₃ nanocomposite.

XRD analysis was conducted to characterize the phase composition, nature, and structure of pure WO₃ and Mn-WO₃ nanocomposites. As depicted in Fig.3a, all the diffraction peaks of pure WO₃ ((002), (020), (200), (112), (022), (222), (220), (132), and (101)) can be indexed to the monoclinic form of WO₃ according to the standard JCPDS No. 83–0951, indicating the purity of the synthesized WO₃ nanoparticles. In the case of Mn-WO₃ nanocomposites, several peaks (120), (152), (310), (410), and (420) were observed and attributed to manganese-based oxide composites or Mn²⁺ in Mn-WO₃ nanocomposite, as per the standard ICDD card No. 320,637. It was noticed that with an increase in Mn dopant concentration, the intensity of the dominant peaks ((200), (020), and (002)) also increased. This observation suggests that Mn incorporation into the WO₃ lattice influences the crystalline structure and phase composition of the nanocomposites, affecting their photocatalytic properties. In summary, XRD analysis confirmed the successful synthesis of pure WO₃ NPs and revealed the presence of manganese-based oxide phases in Mn-WO₃ nanocomposites, indicating that Mn doping can effectively modify the structure and composition of WO₃ for potential photocatalytic applications.

Upon Mn doping, a reduction in the calculated lattice parameter values was observed, which can be attributed to the smaller ionic radius of Mn²⁺ (0.67 Å) compared to W⁶⁺ (0.74 Å). The smaller size of Mn ions enables them to easily access the tungsten (W) crystalline sites and substitute the regular lattice positions of W ions, resulting in Mn-doped WO₃ particles. This substitution leads to a decrease in particle size and an increase in the surface area of the nanocomposite²⁹.The average crystalline sizes of WO₃ and Mn-doped WO₃ with different doping ratios (20, 15, and 5 wt%) were calculated using Scherrer’s equation Eq. (S₁)³⁹. The average crystalline sizes were found to be 30, 18, 22, and 28nm for WO₃, Mn- WO₃ (20%), Mn- WO₃ (15%), and Mn- WO₃ (5%), respectively. This analysis confirms the effect of Mn doping on the crystalline structure and particle size of the synthesized nanocomposites. In conclusion, the incorporation of Mn into WO₃ resulted in a reduction of the lattice parameter values and particle size, leading to an increased surface area of the Mn-WO₃ nanocomposites²⁹. These changes in structural properties can potentially enhance the photocatalytic activity of Mn- WO₃ nanocomposites by providing more active sites for photocatalytic reactions.

The observed results suggest that manganese (Mn²⁺) ions are likely to be successfully incorporated into the regular lattice sites of WO₃ particles. Among the synthesized samples, Mn(15% wt)- WO₃ nanocomposite (denoted as Mn-WO₃) demonstrated the most promising characteristics as a catalyst, based on the comprehensive characterization analyses and preliminary experimental studies. Therefore, Mn- WO₃ was chosen as the optimal catalyst for further experiments, with the aim of exploring its potential applications in photocatalytic processes. The selection of Mn- WO₃ as the best candidate can be attributed to the synergistic effects of Mn doping on the structural, optical, and electronic properties of WO₃. The successful incorporation of Mn into the WO₃ lattice leads to modifications in the band structure, enhanced visible light absorption, and improved charge carrier separation, ultimately contributing to its superior photocatalytic performance.

Nitrogen (N₂) adsorption/desorption analysis was conducted to determine the specific surface area and pore size distribution of the Mn-WO₃ nanocomposite using BET (Brunauer-Emmett-Teller) and BJH (Barrett-Joyner-Halenda) methods. Figure3b depicts the N₂ adsorption/desorption isotherms for the synthesized samples, which were found to be similar to type-III isotherms according to IUPAC classification. The absence of a distinct point B in the type III isotherm indicates that the adsorbed molecules preferentially cluster at the most favorable sites on the macroporous solid surface. The BET method is typically used to estimate the location of the isotherm’s shoulder, known as “Point B,” which signifies the completion of the first monolayer formation⁴⁰.

Table S2 summarizes the surface area, pore volume, and pore size of the synthesized samples. The results indicate a significant decrease in the surface area and pore volume of the Mn-WO₃ nanocomposite, suggesting the presence of mesoporosity in the samples. The structural and morphological analysis further confirmed the formation of a heterojunction between WO₃ and Mn, which contributes to the observed changes in the textural properties of the Mn-WO₃ nanocomposite. Overall, the BET and BJH analyses provided valuable insights into the porous structure and surface properties of the Mn-WO₃ nanocomposite, which can influence its photocatalytic activity by affecting the adsorption of reactants and the accessibility of active sites during photocatalytic reactions. These findings highlight the importance of understanding the role of surface area and porosity in the design and optimization of high-performance photocatalytic materials.

FESEM and TEM technique were used to determine the characterization of the size, properties of surface morphology and textural of the Mn-WO₃ nanocomposite. Figure4a and c present the FESEM images of Mn-WO₃ nanocomposite and pure WO₃. As depicted in Fig.4a, the NPsof pure WO₃ were scattered in various sizes (ranging from 30 to 60nm) and not spherical.

TEM analysis revealed that the NPs exhibited irregularly agglomerated configurations with relatively flat surfaces and spherical agglomerates (Fig.4c). The particle size of the Mn-WO₃ nanocomposite ranged from 20 to 40nm, with a non-uniform rectangular structure. The smaller particle size of the Mn-WO₃ nanocomposite resulted in an irregular tissue morphology and porous surface structure, as indicated by the presence of holes among the particles. Although some areas exhibited optimal dispersion, aggregation was observed in the NPs. EDS was employed to determine the localized elemental information of pure WO₃ NPs (Fig.4b) and Mn-WO₃ nanocomposite (Fig.4d). The pure WO₃ NPs surface was primarily composed of tungsten (78.22% weight%) and oxygen (21.68% weight%). After Mn doping on WO₃, manganese, oxygen, and tungsten were found to be the dominant elements on the nanocomposite surface, with corresponding weight percentages of 14.17%, 18.69%, and 67.14%, respectively. The presence of Mn, W, and O elements in the Mn-WO₃ nanocomposite surface demonstrated the successful doping of Mn particles on WO₃ NPs. The EDS mapping in Fig.5a further confirmed the dispersion of Mn, W, and O elements in the doped nanocomposite. The TEM micrograph (Fig.5b) showed the agglomeration and uniform morphology of the Mn-WO₃ nanocomposite, attributed to the doping of Mn particles in the WO3 structure. Although a few agglomerations of smaller round crystallites were observed, Mn clearly altered the morphology of WO₃ NPs.

FTIR analysis was performed to evaluate the surface characteristics and functional groups of Mn (15%)-WO₃ nanocomposite and pure WO₃ NPs in the range of 400–4000cm^− 1 (Fig.5c). Absorption peaks at 3321, 1625, and 750cm^− 1 were observed in the FTIR spectra of Mn-WO₃ nanocomposite and WO₃. The peak at 3321cm^− 1 was attributed to the stretching modes of O-H groups of adsorbed water, while the intense broad peak at 1625cm^− 1 was assigned to the bending modes of O-H groups. The distinctive peak of O-W-O stretching mode was found in the broad band of 750cm^− 141. The FTIR spectra of Mn (15%)-WO₃ nanocomposite showed significant changes, particularly in the band of 750cm^− 1, indicating the successful incorporation of Mn ions at 15% w.t into the monoclinic structure of WO₃ NPs.

Thermogravimetric analysis (TGA) was conducted to assess the thermal stability of the Mn-WO₃ nanocomposite with 15% Mn content. The results are depicted in Fig.5d. The TGA curves indicate the multi-step weight loss and higher thermal stability of the Mn-WO3 nanocomposite.

No weight loss was recorded at temperatures below 400°C. However, a 2% weight loss was observed in the temperature range of 400–500°C for the Mn-WO₃ nanocomposite. This weight loss may be attributed to the evaporation of adsorbed water and the use of ethanol as a solvent during the synthesis process⁴².

A 5% decline in the weight of the Mn-WO₃ nanocomposite was recorded in the temperature range of 500–700°C. This reduction might be caused by the release of -OH groups, degradation of residual organic matter, and the removal of minor fractions of anions attached to the surface of the synthesized sample⁴³. In the temperature range between 700 and 900°C, a more significant mass loss was observed, likely due to the combustion of the catalyst nanoparticles. Overall, the TGA analysis demonstrates the thermal stability of the Mn-WO3 nanocomposite, with multiple stages of weight loss occurring at different temperature ranges. This information is crucial for understanding the nanocomposite’s behavior under high-temperature conditions, which could impact its performance in various applications, such as photocatalysis.

Experimental design procedure

The experiments were designed with four main variables: DCF concentration (mg L^− 1), reaction time (min), solution pH, and catalyst dose (g L^− 1) to study their impact on DCF removal. The CCD experimental design was used to obtain the optimal conditions for DCF degradation by the Mn-WO₃ nanocomposite. Table S3 provides the details of the experimental design, along with the predicted and experimental data for DCF degradation by Mn-WO₃ nanocomposite via CCD experimental design.

To examine the statistical significance of each variable on DCF removal, the results were analyzed using Analysis of Variance (ANOVA)⁴³. As shown in Table S4, all variables demonstrated a p-value < 0.05, indicating that they were statistically significant and effective in developing the model. This suggested that the predictive model had a strong linear correlation with the experimental data obtained from the trials⁴⁴. The quadratic model equations, expressed in terms of uncoded and coded quantities of factors, are presented in Eqs. S3 and S4, respectively. These equations provide an understanding of the relationship between the variables and the response, DCF degradation. ANOVA analysis (Table2) revealed that the quadratic model had an F-value of 36.32 and a significant Lack of Fit (LOF) value, demonstrating that the model coefficients were significant. The significant F and model coefficients indicated a strong correlation between DCF degradation and the variables (Text DC-3), further validating the model’s reliability⁴⁵.

Based on the results, it can be concluded that the optimization of the variables (DCF concentration, reaction time, solution pH, and catalyst dose) plays a crucial role in maximizing DCF removal efficiency using the Mn-WO₃ nanocomposite. The developed quadratic model can be utilized to predict the optimal conditions for achieving the highest DCF degradation rates under different experimental settings.

To further validate the correlation between the experimental data and the quadratic model, R² values were examined (Text DC-4). A satisfactory correlation was found, supporting the reliability of the model correction. Pareto analysis (Eq. S₅) was then employed to assess the influence of each designated parameter (Pi) or factor (i) on the generated response (Text DC-5).

Contour plots were utilized to visualize the influence of various factors and their interactions on DCF removal, using coefficients from the model (Fig. S2). These plots provided a comprehensive understanding of the relationship between the variables and the response.

The Solver “Add-ins” software was used to identify the optimal experimental conditions for DCF removal, considering the effective parameters. The software suggested the following optimal conditions: an initial DCF concentration of 2 mg L^− 1, a pH of 7, a reaction time of 70min, and a catalyst dose of 2.2g L^− 1. Under these conditions, a complete (100%) removal efficiency of DCF was achieved.

To confirm the rationality of the predicted optimal conditions, four replicate trials were conducted. The results demonstrated that the experimental findings were in good agreement with the predicted optimal conditions, further validating the model. The close agreement between R²adj (0.9799) and R²pred (0.9903) in Fig. S3 indicated the significance of the terms in the model.

Overall, the optimization of DCF removal by the Mn-WO₃ nanocomposite using the CCD experimental design and the subsequent validation of the model through experiments showcase the effectiveness of the proposed approach. These findings provide a robust basis for designing efficient photocatalytic systems for the degradation of DCF and potentially other emerging contaminants in water.

The parameters effect on the performance of DCF degradation

pH solution effect

The photocatalytic degradation process of pollutants is significantly influenced by the solution’s pH, as it affects both the surface charge of the nanocomposite and the chemical structure of organic compounds in the solution⁴⁵. To investigate the impact of different initial solution pH levels (4–10) on DCF removal, a study was conducted under optimal experimental conditions. The results, presented in Fig.6, demonstrate that DCF degradation was higher in acidic solutions compared to alkaline conditions. These findings align with Irandost et al.⁴⁶, who reported an increase in DCF photocatalytic degradation under acidic conditions using N, S co-doped TiO₂@MoS₂ heterojunction catalysts. Considering the acidity coefficient of DCF (pKa = 4.15)⁴⁷, DCF exists in its molecular form at pH values below 4.15 and in its ionic form at pH levels above 4.15. Additionally, the pHpzc of the Mn-WO₃ nanocomposite was determined to be 5.5 (Fig. S4). This indicates that the surface of the Mn-WO₃ nanocomposite is negatively charged at pH levels above 5.5 and positively charged at pH levels below 5.5. In acidic conditions, electrostatic attractions between DCF molecules and the nanocomposite surface increase, thereby enhancing the decomposition rate of DCF.

Another reason for increased DCF degradation efficiency in lower initial pH conditions is that high concentrations of H + ions in acidic environments lead to OH radical generation through reactions with dissolved oxygen⁴⁸. On the other hand, decreased degradation efficiency in alkaline conditions can be attributed to the repulsive electrostatic interaction between the negatively charged surface of the Mn-WO₃ nanocomposite and DCF molecules. This reduces the absorption of DCF on the surface of the Mn-WO₃ nanocomposite, as well as the electron-transfer process between the nanocomposite and DCF molecules, ultimately lowering the photocatalytic degradation efficiency⁴⁹.

In conclusion, the solution’s pH plays a crucial role in the photocatalytic degradation of DCF, with acidic conditions enhancing the decomposition rate through increased electrostatic attractions between the pollutant and the nanocomposite surface, as well as promoting OH radical generation. Understanding these mechanisms can inform the development of more effective photocatalytic processes for the remediation of various environmental contaminants.

Effect of nanocomposite dosage on DCF degradation

The dosage of the nanocomposite is a crucial parameter affecting the degradation efficiency rate of DCF. As shown in Fig.7, the influence of catalyst dosage on DCF degradation efficiency was investigated using Mn-WO₃ nanocomposite in the range of 0.5–2.5g L^− 1 under optimal experimental conditions. The findings demonstrated a significant increase in DCF degradation rate, (C/C₀) degradation ratio, and time intervals as the nanocomposite dosage increased. This enhancement in photocatalytic degradation efficiency of DCF with increasing nanocomposite dosage can be attributed to the higher number of surface-active sites available for the generation of electron-hole pairs^50,51. These electron-hole pairs play a vital role in the degradation of adsorbed contaminants on the nanocomposite surface. Consequently, the increased availability of active sites leads to more efficient pollutant degradation. In summary, optimizing the nanocomposite dosage is essential for maximizing the photocatalytic degradation efficiency of DCF and other contaminants. By increasing the number of surface-active sites, the formation of electron-hole pairs is enhanced, ultimately leading to improved degradation performance. These findings provide valuable insights into the design and application of photocatalytic materials for environmental remediation purposes.

Effects of initial concentration of DCF

The initial concentration of the contaminant is another crucial factor affecting the photodegradation efficiency. Experiments were conducted under optimal conditions using varying initial DCF concentrations (1–10 mg L^− 1) to investigate their impact on photocatalytic degradation. As shown in Fig.8, the degradation rate decreased as the initial DCF concentration increased. This observation can be attributed to several factors. At higher initial concentrations, more DCF molecules adsorb onto the nanocomposite surface, leading to a decrease in available active sites on the catalyst. This results in fewer sites being available for the formation of hydroxyl radicals, which negatively affects the photocatalytic degradation efficiency⁵².

Additionally, at higher contaminant concentrations: (a) More photons are absorbed by the contaminants, leaving fewer photons available for photocatalysis, (b) Although reactive species are stably produced in the reactor, their reaction rate with DCF molecules decreases, and (c) The production of intermediates creates competition between intermediates and reactive radicals for reaction sites. These factors collectively contribute to a reduction in photocatalytic degradation efficiency at higher initial DCF concentrations. Therefore, optimizing the initial contaminant concentration is essential for achieving maximum photocatalytic degradation efficiency in environmental remediation processes.

Control experiment

Preliminary control experiments were conducted to examine DCF photolysis (using visible light irradiation), dark adsorption capacity, and the photocatalytic performance of Mn-WO3 nanocomposite for DCF degradation. These results were then compared with pure WO₃ NPs under optimal conditions. As depicted in Fig.9, DCF photolysis using only LED visible light irradiation had a minor impact on removal efficacy (5.25%). Additional experiments were performed in the absence of light to determine the adsorption effect of Mn-WO₃ nanocomposite on DCF degradation. These findings revealed that only 13% of DCF was removed by Mn-WO₃ nanocomposite under optimal conditions (Fig.9), indicating that adsorption by the photocatalyst did not play a significant role in DCF removal. Furthermore, 53% of DCF was degraded in 70min using pure WO₃ under visible light. Comparatively, Mn doping enhanced the photocatalytic activity of WO₃, with the Mn-WO₃ photocatalyst demonstrating the highest removal efficiency for DCF (99.5%). This enhancement can be attributed to the reduced recombination rate of photogenerated charge carriers and the accelerated electron transfer resulting from Mn doping⁵³.

In conclusion, the Mn-WO₃ nanocomposite displayed superior photocatalytic performance compared to pure WO₃ NPs for DCF degradation under visible light irradiation. These results highlight the potential of Mn-WO₃ nanocomposites for environmental remediation and the importance of optimizing photocatalyst composition to achieve maximum efficiency.

The influence of natural organic matters (NOMs) and competing water anions

The influences of water anions and NOMs on the photocatalytic degradation process are complex, as they can adsorb onto the photocatalyst’s surface active sites and compete with organic contaminants for active radicals⁴⁹. To investigate these effects, the performance of Mn-WO₃ nanocomposite under visible light irradiation was examined in the presence of various anions (SO₄²⁻, NO₃⁻, HCO₃⁻, PO₄³⁻ and Cl⁻) and humic acid (HA), a typical NOM. Additionally, a tap water sample containing different water anions (SO₄²⁻ (125 mg L^− 1), CO₃⁻ (140 mg L^− 1), HCO₃⁻ (60 mg L^− 1), NO₃⁻ (10 mg L^− 1), F⁻ (1.6 mg L^− 1) and Cl⁻ (75 mg L^− 1)) was used to evaluate the impact of water matrices on photocatalytic degradation. Figure10a reveals that all anions exhibited a slight negative effect on DCF degradation. The decrease in degradation rate followed the decreasing order of HCO₃⁻ > SO₄²⁻ > NO₃⁻ > PO₄³⁻ > Cl⁻. Similarly, the findings demonstrated that degradation efficacy in the tap water sample was reduced by just 8.5%. These results align with those reported in other studies⁵⁴. The observed negative effects of anions and NOMs on DCF degradation can be attributed to the competitive adsorption of these species onto the Mn-WO₃ nanocomposite surface, leading to reduced active sites available for DCF degradation. Moreover, these species may scavenge reactive oxygen species (ROS), such as hydroxyl radicals, which are crucial for photocatalytic degradation⁴⁹. Despite the slight reduction in DCF degradation efficiency in the presence of anions and NOMs, the Mn-WO₃ nanocomposite still exhibited strong photocatalytic performance, demonstrating its potential for real-world applications in water treatment.

The influence of various HA concentrations on DCF degradation using Mn-WO₃ nanocomposite was investigated, and the results are presented in Fig.10b. It was observed that DCF degradation was significantly inhibited at higher HA concentrations. Under optimal experimental conditions, the degradation rate of DCF was 90% at 5 mg L^− 1 HA, 79.5% at 30 mg L^− 1 HA, and 56% at 50 mg L^− 1 HA. The inhibition of DCF degradation at higher HA concentrations can be attributed to two main factors. First, HA absorbs incident light, preventing photons from reaching the active sites of the Mn-WO₃ nanocomposite. This reduces the photocatalytic activity of the nanocomposite by limiting the generation of electron-hole pairs⁵⁵. Second, HA scavenges ROS, such as hydroxyl radicals, which are essential for the degradation of organic pollutants in photocatalytic processes. The scavenging of ROS by HA results in a decreased availability of ROS for DCF degradation, leading to lower degradation efficiency⁵⁵. These findings highlight the importance of considering the presence of NOMs, such as HA, when designing photocatalytic water treatment systems. Further research can focus on strategies to minimize the negative impact of NOMs on photocatalytic degradation, such as optimizing the photocatalyst properties or employing advanced oxidation processes to reduce the concentration of NOMs in water sources.

Effect of scavenger

To gain further understanding of the mechanisms involved in the DCF degradation over Mn-WO₃ nanocomposite, free radical quenching tests were performed to identify the reactive species that play a role in the photocatalytic process. Triethanolamine (TEOA), benzoquinone (BQ), and methanol were employed as scavengers for hole (h+), superoxide anion radicals (^·O₂⁻), and hydroxyl radicals (^·OH), respectively. The impact of these scavengers on the DCF degradation rate is depicted in Fig.11. Without the addition of scavengers, the Mn-WO₃ photocatalyst achieved a DCF degradation efficiency of 99.5%. However, upon introducing methanol, a significant drop in DCF degradation was observed (40.1%), signifying that ·OH was the primary reactive species during the degradation process⁵⁶. Furthermore, a 54.3% DCF removal rate was achieved with the addition of TEOA, indicating that h + also contributed to the degradation reaction. In contrast, the presence of BQ resulted in only a slight reduction in the removal rate of DCF (16.3%). This finding suggests that ^·O₂⁻ was not efficiently produced during the degradation process.

The obtained results demonstrate that ·OH, generated through excited holes, plays a crucial role in the photocatalytic degradation of DCF⁵⁷. This highlights the importance of optimizing photocatalytic systems to maximize the production of ·OH radicals, which are highly reactive and effective in degrading organic pollutants. Additionally, the contribution of h + to the degradation process should not be overlooked, as it also contributes to the overall efficiency of the photocatalytic system.

Dechlorination and mineralization of DCF

The mineralization level of DCF in the photocatalytic process using Mn-WO₃ nanocomposite under visible light irradiation was assessed based on TOC removal. As shown in Fig.12, under optimal experimental conditions, the rate of DCF mineralization reached 73%, indicating complete decomposition of DCF. Additionally, by extending the reaction time to 180min, DCF mineralization increased to 88%. This finding suggests that a longer reaction time is necessary for complete mineralization of DCF in the photocatalytic process using Mn-WO₃ nanocomposite under visible light irradiation. The incomplete mineralization of DCF may be attributed to several factors, including the formation of aromatic organic intermediates or by-products during the photocatalytic degradation reaction, or the recombination of charge carriers generated on the photocatalyst surface upon visible light illumination⁵⁸. However, the photocatalytic process using Mn-WO₃ nanocomposite demonstrated a higher DCF mineralization efficiency compared to similar studies. In related studies, DCF mineralization rates of 39% and 19% were achieved using ultrasonic/ozone and O₃/UV/S₂O₈ processes after 40 and 30min of reaction time, respectively^59,60. Additionally, an AOP using magnetic MWCNTs-CoFe₃O₄ NPs to activate peroxymonosulfate resulted in a 50.11% DCF mineralization after 120min⁶¹. A heterogeneous oxidation process utilizing α-MnO₂ nanorods as a catalyst yielded a 40% DCF mineralization rate within 80min⁶². The enhanced mineralization efficiency of DCF in the photocatalytic process using Mn-WO₃ nanocomposite under visible light irradiation can be attributed to the high generation efficiency of ROSs, particularly •OH radicals⁵¹. This emphasizes the significance of optimizing photocatalytic systems to maximize ROS production, ultimately leading to more effective pollutant degradation and mineralization in environmental remediation applications.

The efficiency of a treatment process in removing chlorinated organic compounds is often determined by dechlorination. The dechlorination efficiency of DCF using Mn-WO₃ nanocomposite under visible light irradiation was evaluated by measuring chloride ion concentrations in the final reaction solution under optimal experimental conditions using the Mohr method (Fig.13).

The results indicated that increasing the reaction time led to enhanced dechlorination efficiency of DCF, achieving complete dechlorination after 90min. This complete dechlorination signifies a substantial reduction in the toxicity of the final reaction solution. Consequently, the findings of this study propose that the photocatalytic process using Mn-WO₃ nanocomposite under visible light irradiation is an effective destructive process for both mineralization and dechlorination of DCF. The high dechlorination efficiency observed in this study can be attributed to the photocatalytic activity of Mn-WO₃ nanocomposite, which generates ROSs upon visible light irradiation. These ROS effectively degrade DCF and facilitate the cleavage of carbon-chlorine bonds, ultimately resulting in dechlorination. In conclusion, the Mn-WO₃ nanocomposite demonstrates promising potential for use in environmental remediation processes targeting chlorinated organic contaminants. The ability of this photocatalyst to effectively degrade and dechlorinate DCF under visible light irradiation highlights the importance of optimizing photocatalytic systems for efficient pollutant removal and detoxification in water treatment applications.

Fate and degradation pathway of DCF

Liquid Chromatography-Tandem Mass Spectrometry (LC/MS/MS) analysis was utilized to identify intermediate/by-products formed during the decomposition of DCF using Mn-WO₃ nanocomposite under visible light irradiation. The photocatalytic process was performed under optimal conditions (pH = 7, MnWO₃ = 2.2g L^− 1, DCF = 2 mg L^− 1, and reaction time = 70min) with LED irradiation.

Figure14a and b depict the LC/MS analysis and intermediates produced following DCF degradation in positive mode [M + H+] at m/z 30–240. The analysis identified various intermediates formed during the photocatalytic degradation process, offering insight into the degradation pathway of DCF.

Understanding the intermediates generated during photocatalytic degradation is crucial for evaluating the efficiency and potential environmental impact of the treatment process. By identifying these intermediates, researchers can assess the toxicity of by-products and the extent of pollutant transformation, which is essential for ensuring that the treatment method is not only effective in degrading the target pollutant but also in reducing overall environmental risk.

Furthermore, this information can guide the development of strategies to enhance photocatalytic systems by promoting the formation of less toxic intermediates or facilitating their complete mineralization. In the case of DCF degradation using Mn-WO₃ nanocomposite, the LC/MS/MS analysis contributes valuable information for optimizing the photocatalytic process and advancing its potential application in environmental remediation.

Table3 present 7 intermediary products investigated in LC/MS. Based on the analysis of LC/MS/MS, the decomposition of DCF had two identified intermediary products.

Figure14a presents the chromatogram of the LC-MS analysis during the degradation of DCF, identifying several intermediate products. Some of the identified intermediates include:

1. 1.

   3,5-Dichloro-N-(2-methoxyphenyl) aniline (m/z 91, 35% abundance, RT = 1.15min).

2. 2.

   (2-Amino-phenyl) acetic acid (m/z 99, 45% abundance, RT = 1.44min).

3. 3.

   3,5-Dichloro-4-isopropenyl-phenol (m/z 220, 18% abundance, RT = 1.28min).

4. 4.

   Phenyl-O-tolyl-amine (m/z 222, 35% abundance, RT = 1.15min).

5. 5.

   Maleic acid (m/z 233, 100% abundance, RT = 1.32min).

6. 6.

   Phenol (m/z 247, 80% abundance, RT = 1.26min).

7. 7.

   2-O-Tolylamino-phenol (m/z 258, 25% abundance, RT = 1.26min).

8. 8.

   2,6-Dichloro-N-phenylamin (m/z 274, 15% abundance, RT = 1.18min).

The degradation pathway of DCF can be described as follows:

1. 1.

   DCF molecules (A1) in the aqueous phase are attacked by hydroxyl radicals, leading to the cleavage of C-C and C-H bonds.

2. 2.

   The DCF molecule is transformed into 3,5-Dichloro-N-(2-methoxyphenyl) aniline (A2).

3. 3.

   Intermediate (A2) is further degraded by hydroxyl radicals, forming 2,6-Dichloro-N-phenylamin (A3).

4. 4.

   The intermediate groups are attacked by hydroxyl radicals, yielding 3,5-Dichloro-4-isopropenyl-phenol (A4) and H₂.

5. 5.

   2-O-Tolylamino-phenol (A5) and N₂O₃ are formed.

6. 6.

   The reaction continues, resulting in Phenyl-O-tolyl-amine (A6), (2-Amino-phenyl) acetic acid (A7), and 2-Hydroxy methyl-phenol (A8).

7. 7.

   Finally, Phenol (A9) and Malic acid (A10) are produced.

Malic acid, a saturated short-chain chemical, was identified as the main organic compound found during DCF degradation. Further analysis of LC/MS/MS data (Fig.14b) revealed two prominent intermediate products: m/z 57 (daughter ion of m/z 91, ES + 1.15e4) and m/z 220 (daughter ion of m/z 274, ES + 1.15e4), which are derived from the intermediates with m/z 91 and m/z 274. These findings provide valuable insights into the degradation mechanism of DCF using Mn-WO₃ nanocomposite under visible light irradiation and highlight the potential for further optimizing the photocatalytic process.

Figure14a; Table3 indicate that malic acid and phenol were the primary intermediates of DCF decomposition during the photocatalytic process employing Mn-WO₃ nanocomposite under visible light irradiation. These intermediates exhibit reduced toxicity compared to DCF⁶³. Figure15 illustrates a plausible degradation pathway for DCF in this photocatalytic process based on the molecular fragments identified through LC/MS/MS analysis. It can be concluded that the predominant degradation pathway is driven by hydroxyl radicals. The proposed degradation pathway suggests that hydroxyl radicals play a critical role in the decomposition of DCF. These highly reactive radicals attack the DCF molecule, leading to the formation of various intermediates such as 3,5-Dichloro-N-(2-methoxyphenyl) aniline, 2,6-Dichloro-N-phenylamin, and others. The degradation process continues, ultimately yielding malic acid and phenol as the major intermediates. The formation of malic acid, a relatively less toxic compound, and phenol as primary intermediates during DCF degradation highlights the efficiency of the photocatalytic process using Mn-WO₃ nanocomposite under visible light irradiation. This process not only degrades DCF effectively but also transforms it into less toxic compounds, mitigating the overall environmental impact of DCF contamination. In conclusion, the photocatalytic degradation of DCF using Mn-WO₃ nanocomposite under visible light irradiation offers a promising approach for the treatment of water contaminated with DCF and potentially other organic pollutants. The elucidation of the degradation pathway provides insights into the mechanism of DCF decomposition and paves the way for further optimization of the photocatalytic process to enhance its efficiency and applicability in environmental remediation.

The evaluation of toxicity of treated solution

An important parameter in the photocatalytic degradation process is toxicity analysis. It is known that the degradation of organic pollutants, including pharmaceuticals, can result in the generation of more toxic intermediate products. Therefore, complete transformation or mineralization of organic pollutants to non-toxic chemicals is of utmost importance^64,65. In this regard, D. Magna was selected to investigate the acute toxicity of raw and treated DCF solutions. The number of motionless Daphnia was counted after 24, 48, 72, and 96h, considering the initial number of D.magna (10 Daphnia in each container), as shown in Tables S5 and S6. Table3 presents the lower and upper bounds of the 95% confidence limits and the EC50 values obtained based on the Probit model for toxicity assessment on D.magna. According to the toxicity experiments’ findings in Table3, the EC50 values for 24h, 48h, 72h, and 96h were 88.60, 53.42, 26.53, 14.56, 177.65, 132.23, 114.76, and 85.55 mg L^− 1 for both treated and raw DCF solutions, respectively. Notably, the toxicity value of the treated DCF solution was approximately three-fold lower than that of the raw solution. These results suggest that the photocatalytic process using Mn-WO₃ nanocomposite under visible light irradiation is highly effective in detoxifying DCF. It is confirmed that the photocatalytic process can decompose DCF compounds and their intermediates through hydroxyl radical-induced destruction mechanisms, producing non-toxic simple compounds.

Energy consumption

Energy consumption analysis is a critical operational parameter in photocatalytic processes. High energy consumption is a major challenge in advanced oxidation processes, including photocatalysis, which can hinder the adoption of AOPs. Thus, calculating the energy utilization rate is of paramount importance when selecting a photocatalytic treatment system. The International Union of Pure and Applied Chemistry (IUPAC) Photochemistry Commission recommends using the electrical energy per order (EEO) metric to evaluate energy utilization in oxidation processes, such as photocatalytic treatment systems. According to IUPAC, EEO is defined as the electrical energy (in terms of kW/h) required to degrade a contaminant by 90% in one cubic meter of aqueous solution. The EEO is calculated using Eq.(10):

where EEO is required electrical energy. V, t and P are the solution volume (L), the reaction time (min) and UV lamp power (s) (kW). C_i and C_f are the initial and final concentrations of the contaminant, respectively. The EEO values to logarithmic degradation of DCF at various initial concentrations (1–10 mgL^− 1) with LED lamp (5W) is presented at Fig.16.

The electrical energy required to degrade DCF in 1 m³ of aqueous solution ranged from 2.2 to 21.92 kWh/m³, as shown in Fig.16. These findings indicate that the EEO decreases as the decomposition efficiency increases. Notably, the EEO values obtained in this photocatalytic process are lower than those reported in Table S7. One of the key advantages of the process used in this study is its lower energy requirement compared to similar studies. Therefore, it can be concluded that the Mn-WO₃ nanocomposite-based photocatalytic process under visible light irradiation is an efficient, cost-effective, and economically viable approach for water and wastewater treatment.