Eco-friendly Al-Bi₂WO₆ nanosheets for effective elimination of crystal violet and methylene blue dyes

Diwakar Malla 1,4

Bishal Hamal 1,4

Rakshya Mulmi 2

Neel Kamal Koju 1

Rishi Ram Ghimire 4

Deependra Das Mulmi 1✉ Emaildmulmi@gmail.com

1 Nanomaterials Research Laboratory Nepal Academy of Science and Technology (NAST) 44700 Lalitpur Nepal

2 KIST Medical College and Teaching Hospital 44705 Lalitpur Nepal

3 Environmental Research Laboratory Nepal Academy of Science and Technology (NAST) 44700 Lalitpur Nepal

4 Racah Institute of Physics and the Harvey M. Kruger Family Center for Nanoscience and Nanotechnology the Hebrew University of Jerusalem 919040 Jerusalem Israel

Diwakar Malla^1,4, Bishal Hamal^1,4, Rakshya Mulmi², Neel Kamal Koju¹, Rishi Ram Ghimire⁴, and Deependra Das Mulmi^1*

¹Nanomaterials Research Laboratory, Nepal Academy of Science and Technology (NAST), Lalitpur, 44700, Nepal

²KIST Medical College and Teaching Hospital, Lalitpur, 44705, Nepal

³Environmental Research Laboratory, Nepal Academy of Science and Technology (NAST), Lalitpur, 44700, Nepal

⁴Racah Institute of Physics and the Harvey M. Kruger Family Center for Nanoscience and Nanotechnology, the Hebrew University of Jerusalem, Jerusalem 919040, Israel.

^*Corresponding author: dmulmi@gmail.com

Contact Number: +977-9843058819

Abstract

A hydrothermal technique was followed to prepare an aluminum doped bismuth tungstate (Al-Bi₂WO₆) nanostructure. X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), Energy dispersive X-ray spectroscopy (EDX), Raman scattering spectroscopy (RS), Fourier transform infrared spectroscopy (FTIR), High resolution-transmission electron microscopy (HR-TEM), and Brunauer-Emmett-Teller (BET) method were employed to characterize the synthesized sheet-like nanostructure. The XRD admitted the existence of an orthorhombic structure of synthesized samples. Al-Bi₂WO₆ nanostructure was used as an adsorbent to eliminate crystal violet (CV) and methylene blue (MB) dyes from the aqueous solution. A series of batch experiments were conducted to investigate the removal efficiency of the synthesized nanostructure for CV and MB dyes, varying parameters such as pH, adsorbent dose, contact time, initial concentration, and temperature. The outcomes demonstrate that the most prominent removal percentage of CV and MB dyes at 20 mg/L is 96% and 98%, at 20 and 25 minutes, respectively. The saturated adsorption capacity of Al-Bi₂WO₆ to CV and MB dyes achieved 30.57 mg/L and 15.11 mg/g, from 20 to 50 mg/L, respectively. Moreover, adsorption kinetics and isotherms were best matched with the Langmuir and Pseudo-second-order models, respectively. Further, the thermodynamics factors confirmed that the adsorption process is an exothermic process.

Keywords:

Hydrothermal

Al doped Bi₂WO₆

Organic dyes

Adsorption

Nanostructure

Efficiency

1. Introduction

Growing industrialization is causing environmental pollution in many developing countries like Nepal. The carpet, printing, and textile industries of Nepal have generated employment and foreign currencies (Gautam et al. 2008). However, it has created severe environmental pollution, mainly by polluting water in rivers and land. Dyes are widespread exploited in carpets, textiles, and various other industries as they are essential materials for these sectors (Berradi et al. 2019). Various industrial waste items, including dyes and other toxic substances, are being dumped into rivers without any treatment, harming flora and fauna. The consequences extend beyond ecological damage to affecting human health, as these pollutants can contaminate groundwater, thereby impacting public health (Ahmed et al. 2021). Assessments reveal that carpet and textile wastewater and contaminated soils contain heavy metals and various compounds including organic dyes (Singh et al. 2022, Tchounwou et al. 2012). These substances have repressive consequence on the germination of seeds, heading to a reduction in spermatophyte (Khan &Malik 2018). Many researchers have been interested in organic dyes because of their hard breakdown, durability, high toxicity, and carcinogenicity (Kayani et al. 2025). Crystal violet (CV) dye used in textiles, printing, and various industries has many health hazards, including eye, skin, and respiratory irritation. Prolonged exposure may lead to dermatitis and it can be harmful if inhaled or absorbed (Mani &Bharagava 2016, Mirza &Ahmad 2020, William Au 1978). Similarly, methylene blue (MB), a textile dye, is hazardous to public health and the ecological balance because of its nervous system toxicity (Gillman 2011), capability to DNA damage (Chistyakov et al. 2009), carcinogenicity, and lack of biodegradability (Laszlo Vutskits 2008).

Nanomaterials are widely synthesized for dye adsorption due to their high surface area, tunable porosity, and surface functionality. Dye degradation is a key step in wastewater treatment, since dyes are toxic, stable, and resistant to natural degradation. Different physical, chemical, and biological techniques are used to eliminate dyes. The physical techniques consist of adsorption (Rao et al. 2025), membrane distillation (Banat et al. 2005), and coagulation- flocculation (Riera-Torres et al. 2010). In this case, dyes are not chemically degraded, only transferred to another phase (sludge). These methods remove dyes from water but do not destroy them. In contrast, the chemical techniques comprise of fenton (Rubio-Clemente et al. 2022), ozonation (Gomes et al. 2018), electrochemical oxidation (Wang et al. 2010), reduction (Islam et al. 2020), and photocatalysis (Mulmi et al. 2022). In these phenomena, dyes are either oxidized or reduced, and they breakdown into smaller, less toxic molecules. These methods can fully mineralize dyes to CO₂, H₂O, and inorganic ions (Sarfo et al. 2023). Additionally, biological techniques include bacterial/fungal degradation (Das et al. 2023), enzymatic degradation (Ayub et al. 2025), and algal/phytoremediation (Diaconu et al. 2023). In these phenomena, dyes are degraded using microorganisms, enzymes, and plants. The best part of this method is that it is eco-friendly and low cost. However, the drawbacks of this method include its slow pace and sensitivity to environmental conditions. Adsorption technology surpasses from the rest of them due to its simpler operation, greater effectiveness, low cost, sensitivity to harmful pollutants, and simplicity of design and more environmentally favorable setting (Hu &Hao 2025). Many researchers are hunting to find the best adsorbent materials for removal of dyes from wastewater. High porosity, superior adsorption kinetics, water stability, and reproducible large-scale production are all characteristics of the best adsorbent materials. We were unable to locate a peer-reviewed study that clearly demonstrated aluminum was successfully doped with Bi₂WO₆. Nevertheless, a 2025 SSRN preprint (Longze Chen 2025) reports Al-doped bismuth-tungsten composite coatings (made by laser deposition) with improved visible-light photocatalysis and superhydrophilicity. Because it is a composite coating, it does not prove Al is inside the Bi₂WO₆ lattice, but it shows Al can beneficially modify Bi–W–O systems (Longze Chen 2025). The existing literature explores the study of adsorption using Bi₂WO₆ and various elements doped, such as La, Eu, Mg, Er, I, Cl, Mo, Nb, Ag, Lu, and Fe (Ait Ahsaine et al. 2016, Chen et al. 2019, Chen et al. 2013, Liu et al. 2021, Phuruangrat et al. 2018, Phuruangrat et al. 2017, Song et al. 2011, Wang et al. 2015, Wang et al. 2018, Xu et al. 2014, Zhang et al. 2018). Bi₂WO₆ is made up of an octahedron-shaped (WO₄)²⁻ layer and an ionic layer (Bi₂O₂)²⁺exhibits an orthorhombic structure (Elaouni et al. 2023). Owing to its unique structure, Ni-doped Bi₂WO₆ has demonstrated strong adsorption performance (Wu et al. 2019). Adding suitable metal ions through doping can significantly enhance adsorption activity by altering the structural shape and increasing the specific surface area. Al is frequently used as a post-transition metal element for this purpose. For instance, when Al is introduced into particles like ZnO, the adsorption capability notably exceeds that of the pure particles (Wu et al. 2016). Despite this, there is no existing report except, to study ferroelectric properties (Xu et al. 2022), to our knowledge, on the use of Al-doped Bi₂WO₆ nanostructures and their impact on adsorption performance.

In this work, we used a simple hydrothermal process to create Al- Bi₂WO₆ nanostructures for effective elimination of organic dyes by adsorption techniques. Al- Bi₂WO₆ was synthesized with a high specific surface area and a large number of pores of a suitable size. These characteristics help increase the adsorption performance of nanostructures and facilitate the interaction of organic dyes with active sites. The introduction of Al into Bi₂WO₆ was validated through analyses using EDX, FTIR, and RS. To assess the adsorption capabilities of the Al-Bi₂WO₆, we conducted tests using CV and MB as the target compounds, carried out in the dark. Additionally, we investigated the impact of pH on the solution of dye to test the adsorptive efficiency of Al-Bi₂WO₆. Our findings revealed that the Al-Bi₂WO₆ samples exhibited greater efficacy in adsorption, suggesting their potential as highly efficient adsorbents for eliminating organic pollutants, particularly in acidic wastewater environments. Furthermore, the impacts of contact time, initial dye concentration, adsorbent dosage, and various adsorption mechanisms were studied.

2. Experimental

2.1. Materials and reagents

Sodium Tungstate Dihydrate [Na₂WO₄.2H₂O, Sigma Aldrich, 98%], Bismuth Nitrate Pentahydrate [Bi(NO₃)₃.5H₂O, Sigma Aldrich, 98%], Anhydrous Aluminum Chloride [AlCl₃, Loba Chemie, 98%], Ethanol [Merck, 99%], Methylene Blue [Fisher Scientific, 98%], and Crystal Violet [Himedia, 97%] were used. The analytical-grade reagents were used without prior purification. Additionally, Milli-Q (M.Q.) water was used to make all of the necessary solutions.

2.2. Synthesis of the Al-Bi₂WO₆ nanostructures

Al-Bi₂WO₆ nanostructures were prepared by a hydrothermal technique (details given in SI S1). First, sodium tungstate with a concentration of 0.05 M was dissolved in 100 mL of M.Q. water. It was stirred using a magnetic stirrer for about 10 minutes at 400 rpm. As a result, the solution turned transparent. Then, in the glove box maintained with inert gas, aluminum chloride with a concentration of 7.5 mM was added to the previous solution under vigorous stirring until the solution turned white. Thereafter, 0.1 M bismuth nitrate was added to the solution with continuous stirring at 400 rpm for 30 minutes. Thereafter, the solution was transferred to the Teflon chamber and kept in the autoclave reactor for the hydrothermal process, which was carried out for 5 hours at 180 ºC in the hot air oven. The oven was then shut off and left to cool down naturally at room temperature. Once the solution was cooled, it was centrifuged at 6000 rpm. After forming a white precipitate, the solution was repeatedly rinsed until its pH reached 7. A Whatman filter paper was used to separate the precipitate material later, after which it was annealed for four hours at 100 ^oC and crushed into a powder using a mortar and pestle.

2.3. Characterizations of Al-Bi₂WO₆ nanostructures

The crystal structure of Al-Bi₂WO₆ were studied using a powder X-ray diffractometer (XRD, Bruker D2 Phaser). Different modes of vibration/stretching of molecular bonding and functional groups of the synthesized samples were detected using the FTIR spectrometer (Nicolet is20, Thermo-Fisher Scientific). Different molecules with particular vibrations of compounds were measured by Raman scattering spectroscopy (JASCO, NRS-3100). Sample morphology and size were characterized by FE-SEM (Hitachi S4800) with an operating voltage of 10 kV and current of 10 mA. The lattice spacing, crystallinity, and identification of elements were confirmed by HR-TEM (FEI Technai G2 F30). Nitrogen adsorption–desorption analysis (Autosorb-iQ2, Quantachrome) was employed to evaluate the surface area and pore size distribution of the sample.

2.4. Methodology for adsorption assessments

CV and MB dye solutions with different concentrations were prepared in the laboratory using commercially available dyes in tap water. The concentrations of CV and MB dyes were determined by measuring the absorbance at λ = 590 nm and 663 nm, respectively, of the solutions with a UV-Visual spectrophotometer (Cary 60, Agilent Technologies) and then calculating the concentration of dye solutions.

The amount of adsorbed dyes at time t (Q_t) was estimated by Eq. (1) (Mouni et al. 2018).

$\:\frac{\text{Q}\text{t}=(\text{C}\text{o}\:-\:\text{C}\text{t})}{m}XV$

where V represents the total volume of the liquid in the beaker, m is the mass of the adsorbent used, Q_t is the amount of dye adsorbed per unit mass of adsorbent, C_o is the initial dye concentration, and C_t is the dye concentration in the solution at time t. The equilibrium concentration C_e in the liquid phase was substituted for C_t in Eq. (1) to determine the quantity of adsorbed dye at equilibrium (Q_e). Furthermore, Eq. (2) was used to compute and assess the removal efficiency (Dhobi et al. 2024).

Removal Efficiency =

$\:\frac{{\text{C}}_{\text{o}}-\:{\text{C}}_{\text{t}}}{{\text{C}}_{\text{o}}}\:\times\:100\:\:$

(2)

2.5. Adsorption analysis

2.5.1. Kinetics study

Kinetic data were fitted to pseudo-first-order (PFO) (Yang et al. 2021), pseudo-second-order (PSO) (Cordova Estrada et al. 2021), and intraparticle diffusion (IPD) (Huang et al. 2018) models to analyze the kinetics of the adsorption process. The expressions of the PFO, PSO, and IPD kinetic models are as follows:

PFO

$\:\text{ln}\:({\text{Q}}_{\text{e}}-\:{\text{Q}}_{\text{t}})=\text{l}\text{n}\:{\text{Q}}_{\text{e}}-\:{\text{k}}_{1}\text{t}$

(3)

PSO

$\:\frac{\text{t}}{{\text{Q}}_{\text{t}}}=\:\frac{1}{{\text{k}}_{2}.{\text{Q}}_{\text{e}}^{2}}+\:\frac{\text{t}}{{\text{Q}}_{\text{e}}}\:\:\:\:\:\:\:\:$

(4)

IPD

$\:{\text{Q}}_{\text{t}}=\:{\text{k}}_{\text{i}\text{n}\text{t}}{\text{t}}^{\frac{1}{2}}+\text{C}$

(5)

where k_int (mg.g^− 1.min^− 1/2) is the IPD rate constant, C (mg/g) is the boundary layer thickness constant, k₁ (min^− 1) is the PFO rate constant, and k₂ (g.mg^− 1.min^− 1) is the PSO rate constant. A non-linear regression technique was used to determine all kinetic parameters.

2.5.2 Isotherm studies

Initial concentration ranges of 20–50 mg. L^− 1 were used to study the equilibrium adsorption isotherms of CV and MB dye molecules on the surface of Al- Bi₂WO₆. The obtained adsorption equilibrium data were analyzed using the Langmuir (Yang et al. 2021) and Freundlich isotherm models, represented by equations (6) and (7), respectively.

Langmuir

$\:\frac{{\text{C}}_{\text{e}}}{{\text{Q}}_{\text{e}}}=\:\frac{{\text{C}}_{\text{e}}}{{\text{Q}}_{\text{m}}}+\:\frac{1}{{\text{Q}}_{\text{m}}.{\text{K}}_{\text{L}}}$

(6)

Freundlich

$\:\text{log\:}{\text{Q}}_{\text{e}}=\text{log}\:{\text{K}}_{\text{f}}+\text{log}\frac{{\text{C}}_{\text{e}}}{{\text{n}}_{\text{f}}}$

(7)

Where, Q_m (mg/g) denotes the maximum adsorption capacity corresponding to complete monolayer coverage at the highest dye concentration applied in the experiment, and K_L(L.mg^− 1) is the Langmuir constant, which reflects the adsorption rate. In the Freundlich model, K_f (mg·g⁻¹) indicates the adsorption capacity, while n_f describes the adsorption intensity.

2.5.3. Thermodynamics studies

To comprehend the adsorption behavior of dyes on the surface of Al- Bi₂WO₆, thermodynamic characteristics including Gibbs free energy (ΔG°), change in enthalpy (ΔH°), and change in entropy (ΔS°) are crucial. These factors were computed using experimental data collected at various temperatures (Cordova Estrada et al. 2021).

The equations for the adsorption thermodynamics are given as follows:

$\:\text{ln}\:{K}_{d}=\:-\frac{{\varDelta\:G}^{o}}{RT}=\:-\frac{{\varDelta\:H}^{o}}{RT}+\:\frac{{\varDelta\:S}^{o}}{R}$

Where K_d(L/mg) is the distribution constant given as,

$\:{\text{K}}_{\text{d}}=\:\raisebox{1ex}{${\text{Q}}_{\text{e}}$}\!\left/\:\!\raisebox{-1ex}{${\text{C}}_{\text{e}}$}\right.$

R (8.314 J.mol^− 1K^− 1) is the universal gas constant and T(K) is the absolute temperature of the given solution.

3. Results and discussions

3.1. XRD studies

Figure 1 shows the XRD patterns of Al-doped Bi₂WO₆, exhibiting sharp and well-defined diffraction peaks that confirm the strong crystallinity of the orthorhombic phase. The characteristic peaks appear at 2θ values of 28.35°, 32.86°, 47.15°, 55.92°, 58.56°, 66.38°, 76.08°, and 78.37°, which correspond to the diffraction planes (131), (200), (202), (133), (262), (400), (333), and (204), respectively. These diffraction peaks are in good agreement with the standard JCPDS card No. 79-2381, with no additional peaks detected, indicating the material’s high purity. On consideration of the intense peak of the (1 3 1) plane, the lattice spacing is found to be 0.314 nm, which is very close to values reported in earlier studies (Sayed et al. 2025, Zhang et al. 2022).

3.2. FE-SEM studies

We observed the formation of a two-dimensional nanosheet-like structure of Al-Bi₂WO₆ (detail given in SI S2). The size was measured using ImageJ software, and the data was analyzed. The average values of width, length, and thickness of Al-Bi₂WO₆ are 12.26 nm, 59.85 nm, and 11.35 nm, respectively.

3.3. Raman spectroscopy studies

Figure 2 represents Raman spectra of Al-Bi₂WO₆. It shows characteristic peaks distributed between 100–330 cm⁻¹ and 600–900 cm⁻¹, allowing for identification of Bi-O and WO₄ vibrational modes and the antisymmetric bridging mode of WO₆. These peaks, particularly the ones around 720 cm⁻¹, 807 cm⁻¹, and 838 cm⁻¹, help characterize the structural and vibrational properties of bismuth tungstate, including its tungstate species and symmetry (Wach 1995). The peaks between 100 and 330 cm⁻¹ are assigned to vibrations of Bi-O and WO₄ units within the Bi₂WO₆ structure (Zou et al. 2015). Translational modes of Bi and W are observed at 172 cm^− 1, and bending of WO₆ and (Bi₂O₂)²⁺ are observed at a Raman shift of 320 cm^− 1(Mączka et al. 2011).

3.4. FTIR studies

Figure 3 indicates the transmittance mode of the FTIR spectrum of Al-Bi₂WO₆. We can observe a steep decrease of the spectrum below 1000 cm^− 1, which shows that most spectra are absorbed in this range. The absorption peaks around 400–1000 cm^− 1 are due to metal oxide bonds. There are no other peaks, which indicates no impurities in the sample. The vibration of the bonds of W-O is at 820 cm^− 1 and 732 cm^− 1 due to the asymmetric mode stretching of WO₆ mainly, apical oxygen and equatorial oxygen, respectively (Phuruangrat et al. 2014). The metallic bond Bi-O formed at the wavenumber less than 500 cm^− 1 is not available due to the limitation of the range of the instrument.

3.5. HR-TEM/EDS studies

Figure 4 shows the morphology of Al-Bi₂WO₆ using HR-TEM. It depicts the clear observation of lattice fringes. The interplanar spacing of Bi₂WO₆ (131) and (200) planes were estimated to be 0.314 nm and 0.27 nm, respectively (Lei Xiang &Quan-Ying Cai 2018, Sayed et al. 2025). The d value obtained from the XRD analysis is consistent with the HR-TEM values. The interplanar spacing of the Al plane (200) was found to be 0.2 nm, which also confirmed the incorporation of the Al content in the Bi₂WO₆. We also determined the lattice spacing d = 0.375 nm, representing the (020) plane of WO₃ (Li et al. 2020). Figure 5 displays the EDS picture of Al-Bi₂WO₆.

The compositions of Al, Bi, W, and O were seen in the image. According to the pictorial data, the concentration of Al in total particle composition is lower than the other components. Figure 5(a) of the HR-TEM/EDS picture depicts the Al-Bi₂WO₆ as a whole, whereas Fig. 5 (b-e) show Al, Bi, W, and O compositions in the Al-Bi₂WO₆ nanostructures.

3.6. N₂ adsorption studies

The porous structure of the Al- Bi₂WO₆ was investigated using Barrett-Joyner-Halenda (BJH) pore size distribution, N₂ adsorption-desorption isotherms, and BET surface area analysis. The IUPAC type IV isotherm is supported by the N₂ adsorption-desorption isotherm curve, as seen in Fig. 6. The appearance of mesopores between 2 and 50 nm is suggested by the presence of a hysteresis loop in the comparatively high-pressure range (Fu et al. 2006). The corresponding pore-size distribution is of type H3, suggesting the existence of slit pores as a result of the aggregation of nanostructures, consistent with the findings from FE-SEM analysis. The surface area, pore volume, and pore diameter size of Al-Bi₂WO₆ are 74.140 m²/g, 0.211 cc/g, and 3.564 nm, respectively. Notably, Al- Bi₂WO₆ has the highest pore capacity (0.211 cc/g), indicating that it may be useful for organic dye dispersion within the porous structure.

3.7. Adsorption Interaction

The adsorption interaction of CV and MB dyes in the aqueous solution was studied by employing Al-Bi₂WO₆ (details given in SI S3). The schematic diagram (S3) illustrates a standard procedure for measuring dye adsorption using UV-Visual spectroscopy. Initially, an adsorbent is introduced into a dye solution to initiate the adsorption process. The mixture is stirred at 300 rpm to ensure uniform contact between the dye molecules and the adsorbent surface. At predetermined time intervals, a 2 ml aliquot is extracted and centrifuged at 6000 rpm for 5 minutes to separate the solid adsorbent from the liquid phase. The supernatant is then analyzed using a UV-Visual spectrophotometer to measure absorbance, which correlates with the remaining dye concentration. This method enables precise monitoring of adsorption kinetics and efficiency over time, providing critical data for evaluating adsorbent performance.

The impacts of adsorbent dose, contact time, initial dye concentration, and pH were further investigated in relation to the adsorption properties.

3.7.1. Effect of adsorbent dose

The amount of adsorbent utilized is a crucial component in the adsorption process. This is due to the fact that the quantity of adsorbent has a direct impact on the adsorbent powder's surface area, which in turn affects the adsorption efficiency. Figure 7 illustrates the varying amounts of the adsorbent Al-Bi2WO₆ used (from 0.025 to 0.1 g doses with an increment of 0.025 g), with separate doses determined for CV and MB dyes. The quantities of adsorbent ranged from 0.5 to 2 g/L. It is observed that as the dosage of adsorbent increases, the effectiveness of adsorption also increases. Consistent results were observed by Gong et. al (2005) (Gong et al. 2005), concluding that there is a presence of active sites on the surface of the adsorbent, making it easier for the dyes to be adsorbed onto the Al-Bi₂WO₆ surface. So, 2 g/L is the ideal adsorbent dose for MB dye. The CV dye's performance at 1.5 g/L and 2 g/L adsorbent doses, however, is almost identical, suggesting that at higher adsorbent doses, some of the Al-Bi₂WO₆ surface is left exposed as a result of adsorbent particle aggregation, which reduces the actual utilization efficiency (Li et al. 2020). It is therefore advised to use 1.5 g/L of CV dye adsorbent and 2 g/L of MB dye.

Figure 7(a) demonstrates a rapid increase in removal efficiency, reaching near-complete removal (95%) at an adsorbent dose of 0.05 g. This suggests a strong affinity between CV molecules and the adsorbent surface, likely due to favorable electrostatic interactions or molecular compatibility. The early plateau in efficiency indicates that the adsorption sites become saturated quickly, making CV removal highly efficient even at lower doses. In contrast, MB requires a higher adsorbent dose (0.075 g) to achieve similar removal efficiency (93%). The more gradual slope of the MB curve implies slower adsorption kinetics or weaker binding interactions. Additionally, the slightly larger error bars observed in MB measurements suggest greater variability in adsorption performance, potentially due to differences in molecular structure, solubility, or competition for active sites. These findings highlight the superior adsorption behavior of CV under the tested conditions, making it a more favorable target for low-dose treatment strategies. For MB, optimization of adsorbent properties such as surface area, functional groups, or contact time—may be necessary to enhance removal efficiency. Nevertheless, the adsorbent shows promising performance for both dyes, but its effectiveness is significantly higher for CV. This insight can guide future design and application of adsorbents in wastewater treatment, particularly in tailoring materials for specific contaminants.

3.7.2. Effect of contact time

Figure 8(a) and 8(b) show the removal efficiency of CV and MB dyes with time for different doses, respectively. Removal efficiencies for the 0.075 g and 0.1 g doses overlap in the case of CV, however this is not the case in MB. Further, we observed the removal efficiency dependent on the adsorbent doses. Higher doses (0.1 g) result in faster and more complete removal, reaching nearly 100% efficiency. The removal efficiencies of CV and MB dyes are 98% and 96%, respectively. One can clearly see both CV and MB dye solutions became transparent for adsorbent doses of 1.5 g/L and 2 g/L of 20 mg/L dye solutions, respectively. For the CV and MB dyes, the removal efficiency process started off quickly for 10 minutes and 15 minutes, respectively, but it then slowed down and reached equilibrium in 20 minutes and 25 minutes, respectively.

3.7.3. Effect of initial concentration of dye with time

Another important factor in the adsorption process is the duration of contact between the dye and the Al-Bi₂WO₆ surface. Regardless of the starting concentrations, Fig. 9 shows that the adsorption process splits into two separate stages. The first stage of adsorption for both CV and MB dyes happened quickly within 10 and 15 minutes, respectively. After then, the process slowed down and took 20 minutes for CV and 25 minutes for MB to reach equilibrium. This phenomenon is explained by the large number of active sites on the Al-Bi₂WO₆ surface, which strengthens the adsorption's driving force. Furthermore, there might have been an electrical interaction between the cationic dye components and the Al-Bi₂WO₆ surface. The dye adsorption capacity approached equilibrium in the second phase due to the dwindling number of active sites on the adsorbent surface. Additionally, the value of Q_t (adsorption capacity) increased to 30.17 mg/g for CV and 15.11 mg/g for MB when the initial dye concentration was raised from 20 to 50 mg/L. The underlying explanation of this occurrence may be the higher driving force created by this concentration increase, which makes it easier for dyes from the aqueous solution to enter the adsorbent's interior pores [54] and promote dye adsorption onto Al-Bi₂WO₆.

3.7.4. Effect of pH

The adsorbent's molecular structure and active surface sites are altered as a result of the solution's acidic and basic ions reacting with it. Experiments were carried out at room temperature in the pH range of 2 to 10 in order to evaluate the effect of the initial pH on the adsorption of CV and MB dyes. The starting concentration was kept constant at 20 mg/L throughout these tests, and the adsorbent dosage was fixed at 1.5 g/L for CV dye and 2 g/L for MB dye. The obtained impacts of pH on both dyes, CV and MB, respectively, are illustrated in Fig. 10. We found that the removal efficiency is the highest in acidic medium for both CV and MB dye. The removal efficiency of CV and MB, increases in an acidic medium because the acidic conditions protonate the functional groups on the adsorbent surface, making them more negatively charged and enhancing electrostatic attraction to the positively charged dye molecules. This increases the affinity of the dye for the adsorbent, leading to greater adsorption and thus higher removal efficiency (Aldegs et al. 2008).

3.8. Adsorption interaction mechanism

3.8.1. Kinetic studies

To analyze the kinetics of adsorption and gain deeper insights into the adsorption mechanism, various kinetic models such as the PFO, PSO, and IPD models were employed. Figures 11 and 12 display the fitting outcomes for the PFO and PSO models, respectively. A set of kinetic variables is presented in Table 1 for CV and MB dyes, encompassing equilibrium adsorption capacity (Q_e.cal), rate constant (K), and correlation coefficient (R²). The results indicate that the correlation coefficients (R²) associated with the PSO model is remarkably close to 1, exceeding those of the PFO model. Furthermore, the calculated equilibrium adsorption capacities (Q_e.cal) obtained from the PSO model closely match the experimental results.

Table 1
CV (mg.L^− 1)	Q_e.exp	PFO			PSO			IPD
CV (mg.L^− 1)	Q_e.exp	Q_e1.cal	k₁	R²	Q_e2.cal	k₂	R²	K_int.1	K_int.2	K_int.3
20	12.9112	1.0247	0.1174	0.7550	12.9685	0.5722	0.9999	5.6282	0.0731	0.0059
25	16.1170	1.0071	0.0592	0.4107	16.1943	0.7180	0.9995	7.0451	0.0289	0.0108
30	19.2626	3.9149	0.1387	0.9225	19.6194	0.1467	0.9999	8.1048	0.2234	0.0708
40	25.0210	3.9149	0.1190	0.9375	26.1780	0.0469	0.9997	9.9065	0.7840	0.2592
50	30.5766	13.690	0.1369	0.9132	31.9898	0.0381	0.9996	12.216	0.8239	0.3641
MB (mg.L^− 1)
20	9.7740	1.4737	0.1010	0.8155	9.8961	0.2338	0.9999	3.9300	0.1605	0.0132
25	11.9649	3.9520	0.0983	0.9362	12.4455	0.0719	0.9999	4.4391	0.4355	0.0742
30	13.3246	1.5670	0.0715	0.6643	13.9567	0.0569	0.9998	4.7580	0.5526	0.0556
40	14.0146	2.3655	0.0597	0.6314	14.3906	0.0961	0.9996	5.1930	0.4448	0.0568
50	15.1112	1.6173	0.0732	0.6593	15.3704	0.2290	0.9999	6.4799	0.0402	0.0348

Consequently, the adsorption kinetics' fitting results indicate that the PSO kinetic model is followed when CV and MB dyes are adsorbed onto Al-Bi₂WO₆.

The IPD model was also used to investigate the adsorption mechanism of the MB and CV dyes on the Al-Bi₂WO₆ sample. The IPD model findings are shown in Fig. 13, where k_int.1 > k_int.2 > k_int.3, suggesting that the adsorption took place in three separate processes. CV and MB dyes were quickly driven by concentration to the sample's outer surface during the first stage, which is known as rapid adsorption. When the exterior surface's active sites are occupied, the second stage also referred to as intraparticle diffusion occurs. To delve into the adsorption mechanism of CV and MB dyes on the Al-Bi₂WO₆ sample, the IPD model was also employed. Figure 13 displays the results of the IPD model, where k_int.1 > k_int.2 > k_int.3, indicating that the adsorption occurred in three distinct steps. In the first stage, which is characterized as immediate adsorption, CV and MB dyes were swiftly driven by concentration to the outer surface of the sample. The second stage, known as intraparticle diffusion or gradual adsorption, occurs when the active sites on the external surface become occupied. Dye progressively seeped into the sample's interior pores and stuck to its inner surface. The third stage is known as the equilibrium stage, during which the slowdown of intraparticle diffusion is caused by the low dye solution concentration. Higher concentrations sped up the diffusion of dye molecules within the adsorbent's internal pores, as evidenced by the rate constant (k_int), which increased concurrently with the initial concentrations of CV and MB. This result suggests that the adsorption process was not solely governed by a single rate-limiting mechanism but involved a combination of these stages to achieve adsorption equilibrium (Lei et al. 2017).

3.8.2. Isotherm studies

Figure 14 and 15 depict the observed result of CV and MB dye concentrations at equilibrium (C_e) and the equilibrium adsorbed amount (Q_e) for linear fitting curves, which were applied to Langmuir and Freundlich isotherm models, respectively. The variables and correlation coefficients for these isotherm models are listed in Table 2 for CV and MB dyes. The linear correlation for the adsorption of dyes in Al-Bi₂WO₆ closely follows the Langmuir isotherm model. The correlation coefficient values (R² = 0.9663) for CV and (R² = 0.9985) for MB dyes are higher than those obtained from the Freundlich model (R² = 0.8962) for CV and (R² = 0.9776) for MB dyes, respectively. Furthermore, the experimental adsorption capacity aligns well with the Langmuir isotherm calculations, yielding maximal monolayer adsorption capacities of 31.3990 mg/g for CV and 15.2507 mg/g for MB dyes. Consequently, it can be concluded that the adsorption process of CV and MB on the Al-Bi₂WO₆ surface is primarily characterized by monolayer physical adsorption and is facilitated through electrostatic interactions (Wang et al. 2016). Binding and aggregation of target species occur in physical adsorption due to attraction forces such as Van der Waals forces. Van der Waals interactions and electrostatic attractions are two types of physical adsorption. Van der Waals forces are always present, but electrostatic forces are only important in non-neutral or ionic formations (Pourhakkak et al. 2021).

Table 2
Q_max.exp mg.g^− 1		Langmuir			Freundlich
Q_max.exp mg.g^− 1		Q_max (mg.g^− 1)	K_L (L.mg^− 1)	R²	n_f	K_f (mg.g^− 1)	R²
CV	31.3990	44.4642	0.8643	0.9663	2.0232	19.7677	0.8962
MB	15.2507	15.2898	4.3039	0.9985	13.7817	12.3907	0.9776

3.8.3. Thermodynamic studies

To assess the impact of solution temperature on the adsorption of CV and MB dyes onto the Al-Bi2WO₆, the study examined a range in temperature from 25°C to 85°C. The outcomes are presented in Fig. 16, which shows that the adsorption efficiency of both dyes exhibited a slight decrease, dropping from 96% to 93% for CV dye and from 97% to 95% for MB dye as the temperature rose. This observation recommends that the primary mode of adsorption of Al-Bi2WO₆ for these dyes is predominantly physical adsorption. The observed drop in adsorption effectiveness is caused by a little increase in solution disorder as the temperature rises, which invites dye molecules adsorbed on the Al-Bi₂WO₆ surface to enter the solution. Figure 17 represents a plot of Van't Hoff equation for the adsorption of CV and MB dyes onto Al-Bi2WO6. The thermodynamic factors (ΔH°, ΔS°, and ΔG°) calculated from the experimental data are provided in Table 3 for CV dye and MB dye solutions, respectively. The Van't Hoff plot was utilized to determine ΔH° and ΔS° for the adsorption of these dyes. It is evident from the results that all the calculated values of ΔH°, ΔS°, and ΔG° are less than zero. This indicates that the adsorption of these dyes onto the surface of the Al-Bi2WO₆ is an exothermic. In other words, the adsorption of CV and MB dyes on the Al-Bi2WO₆ is thermodynamically favorable, and the process releases heat while proceeding spontaneously in an orderly manner [62]. Additionally, when the temperature rose, the value of ΔG° increased a little and all belonged to − 20∼0 kJ/mol, indicating that the adsorption process is a physical one and is more favorable to low temperatures [38].

Table 3
	ΔH^o (kJ mol^− 1)	Δ S^o (kJ mol^− 1)	Δ G^o (kJ mol^− 1)
CV	-12.8099	-17.4873	298K	318K	338K	358K
CV	-12.8099	-17.4873	-7.5875	-7.4522	-6.6996	-6.55573
MB	-9.7079	-7.1335	-7.6154	-7.4500	-7.3147	-7.0922

4. Conclusion

The facile synthesis of the nanostructured Bi₂WO₆ doped with Al was achieved by the hydrothermal method. Al-Bi₂WO₆ was then employed as an adsorbent to remove the dyes CV and MB from the aqueous solution. Batch experiments were conducted to investigate the removal efficiency of CV and MB dyes in different environmental conditions. The PSO model is validated by the adsorption kinetic data. The IPD model shows that there were other rate-limiting mechanisms than the adsorption process. For this investigation, the Langmuir model is close to the given Al-Bi₂WO₆, which indicates that the adsorption process is a monolayer physical adsorption that includes the Van der Waals force of attraction and the electrostatic attraction. The maximum removal efficiencies of synthesized Al-Bi₂WO₆ were 96% and 98% for CV and MB dye in 20 and 25 minutes, respectively. And the saturated adsorption capacity reached 30.57 mg/g and 15.11 mg/g for CV and MB dye solutions, respectively. Further, the thermodynamics study reveals that the removal efficiency was effective for low temperatures for both dyes. This nanostructured Al-Bi₂WO₆ efficiently removes cationic dyes CV and MB. This implies that the adsorption process was mostly due to chemical contact between the adsorbent and dye.

Acknowledgements

One of the authors DDM would like to acknowledge Dr. Lok Kumar Shrestha, International Center for Materials Nano architectonics, National Institute for Materials Science (NIMS), Japan, for his invaluable support in characterizing samples.

Author contributions

D.M., B.H., R.M., and N.K.K. carried out the experiment, prepared the materials, created the figures, and wrote the initial draft of the paper. R.R.G. oversaw and edited the manuscript. D.D.M. guided the project and reviewed the manuscript.

Ethical approval

This study did not require ethical approval because it did not involve human participants, personal data, or animal subjects. The methodology complied with the ethical standards of Nepal Academy of Science and Technology and followed international research guidelines.

Consent to Participate

Not applicable

Consent to Publish

Not applicable

Competing interests

No conflicting interests are disclosed by the writers.

Data availability

On request, data will be made available.

Funding

No fundings or other support were received.

Electronic Supplementary Material

Below is the link to the electronic supplementary material

Supplementary Material 1

Additional Files

Additional file 1

5. References

Ahmed J, Thakur A, Goyal A (2021) Industrial Wastewater and Its Toxic Effects. In: Shah MP, Editor (eds) Biological Treatment of Industrial Wastewater. The Royal Society of Chemistry, pp 1–14

Ait Ahsaine H, Ezahri M, Benlhachemi A, Bakiz B, Villain S, Guinneton F, Gavarri JR (2016) Novel Lu-doped Bi2WO6 nanosheets: Synthesis, growth mechanisms and enhanced photocatalytic activity under UV-light irradiation. Ceram Int 42:8552–8558

Aldegs Y, Elbarghouthi M, Elsheikh A, Walker G (2008) Effect of solution pH, ionic strength, and temperature on adsorption behavior of reactive dyes on activated carbon. Dyes Pigm 77:16–23

Ayub A, Wani AK, Chopra C, Sharma DK, Amin O, Wani AW, Singh A, Manzoor S, Singh R (2025) : Advancing Dye Degradation: Integrating Microbial Metabolism, Photocatalysis, and Nanotechnology for Eco-Friendly Solutions. Bacteria 4

Banat F, Al-Asheh S, Qtaishat M (2005) Treatment of waters colored with methylene blue dye by vacuum membrane distillation. Desalination 174:87–96

Berradi M, Hsissou R, Khudhair M, Assouag M, Cherkaoui O, El Bachiri A, El Harfi A (2019) : Textile finishing dyes and their impact on aquatic environs. Heliyon 5, e02711

Chen H, Zhang C, Pang Y, Shen Q, Yu Y, Su Y, Wang J, Zhang F, Yang H (2019) Oxygen vacancy regulation in Nb-doped Bi(2)WO(6) for enhanced visible light photocatalytic activity. RSC Adv 9:22559–22566

Chen R, Hu CH, Wei S, Xia JH, Cui J, Zhou HY (2013) Synthesis and Activity of Ag-Doped Bi. Mater Sci Forum 743–744:560–566. %3C sub %3E 2 WO %3C sub %3E 6 Photocatalysts

Chistyakov VA, Sazykina MA, Kolenko MA, Chervyakov GG, Usatov AV (2009) Methylene blue as a suppessor of the genotoxic effect of ultraviolet radiation with a wavelength of 300–400 nm. Russian J Genet 45:304–307

Cordova Estrada AK, Cordova Lozano F, Lara Díaz RA (2021) Thermodynamics and Kinetic Studies for the Adsorption Process of Methyl Orange by Magnetic Activated Carbons. Air, Soil and Water Research 14

Das S, Cherwoo L, Singh R (2023) Decoding dye degradation: Microbial remediation of textile industry effluents. Biotechnol Notes 4:64–76

Dhobi SH, Neupane D, Koirala S, Das Mulmi D (2024) : Waste tea as absorbent for removal of heavy metal present in contaminated water. Heliyon 10, e39519

Diaconu LI, Covaliu-Mierla CI, Paunescu O, Covaliu LD, Iovu H, Paraschiv G (2023) : Phytoremediation of Wastewater Containing Lead and Manganese Ions Using Algae. Biology (Basel) 12

Elaouni A, El Ouardi M, BaQais A, Arab M, Saadi M, Ait Ahsaine H (2023) Bismuth tungstate Bi(2)WO(6): a review on structural, photophysical and photocatalytic properties. RSC Adv 13:17476–17494

Fu H, Zhang L, Yao W, Zhu Y (2006) Photocatalytic properties of nanosized Bi2WO6 catalysts synthesized via a hydrothermal process. Appl Catal B 66:100–110

Gautam R, Baral S, Herat S (2008) : Opportunities and challenges in implementing pollution prevention strategies to help revive the ailing carpet manufacturing sector of Nepal. Resources, Conservation and Recycling 52, 920–930

Gillman PK (2011) CNS toxicity involving methylene blue: the exemplar for understanding and predicting drug interactions that precipitate serotonin toxicity. J Psychopharmacol 25:429–436

Gomes J, Lopes A, Bednarczyk K, Gmurek M, Stelmachowski M, Zaleska-Medynska A, Quinta-Ferreira M, Costa R, Quinta-Ferreira R, Martins R (2018) : Effect of Noble Metals (Ag, Pd, Pt) Loading over the Efficiency of TiO2 during Photocatalytic Ozonation on the Toxicity of Parabens. ChemEngineering 2, 4

Gong R, Ding Y, Liu H, Chen Q, Liu Z (2005) Lead biosorption and desorption by intact and pretreated Spirulina maxima biomass. Chemosphere 58:125–130

Hu Q, Hao L (2025) : Adsorption Technologies in Wastewater Treatment Processes. Water 17

Huang Y, Lee X, Grattieri M, Macazo FC, Cai R, Minteer SD (2018) A sustainable adsorbent for phosphate removal: modifying multi-walled carbon nanotubes with chitosan. J Mater Sci 53:12641–12649

Islam MT, Hasan MM, Shabik MF, Islam F, Nagao Y, Hasnat MA (2020) : Electroless deposition of gold nanoparticles on a glassy carbon surface to attain methylene blue degradation via oxygen reduction reactions. Electrochim Acta 360

Kayani KF, Mohammed SJ, Mustafa MS, Aziz SB (2025) Dyes and their toxicity: removal from wastewater using carbon dots/metal oxides as hybrid materials: a review. Mater Adv 6:5391–5409

Khan S, Malik A (2018) Toxicity evaluation of textile effluents and role of native soil bacterium in biodegradation of a textile dye. Environ Sci Pollut Res 25:4446–4458

Laszlo Vutskits AB, Klauser P, Gascon E, Dayer AG, Kiss JZ, Muller D, Licker MJ, Denis R, Morel (2008) Adverse Effects of Methylene Blue on the Central Nervous System. Anesthesiology 108:684–692

Lei C, Pi M, Zhou W, Guo Y, Zhang F, Qin J (2017) Synthesis of hierarchical porous flower-like ZnO-AlOOH structures and their applications in adsorption of Congo Red. Chem Phys Lett 687:143–151

Lei Xiang LC, Mo C-H, Zheng L-M, Yu Z-X, Li Y-W, Cai Q-Y (2018) HL, Wei-Dong Yang, Dong-Mei Zhou, and Ming-Hung Wong : Facile synthesis of Ni-doping Bi2WO6 nano-sheets

with enhanced adsorptive and visible-light

photocatalytic performances J Mater Sci 53, 7657–7671

Li G, Wang Y, Bi J, Huang X, Mao Y, Luo L, Hao H (2020) : Partial Oxidation Strategy to Synthesize WS(2)/WO(3) Heterostructure with Enhanced Adsorption Performance for Organic Dyes: Synthesis, Modelling, and Mechanism. Nanomaterials 10

Liu Y, Huang L, Fang Y, Zhu X, Dong S (2021) Achieving ultrahigh electrocatalytic NH3 yield rate on Fe-doped Bi2WO6 electrocatalyst. Nano Res 14:2711–2716

Longze Chen XQ, Xiaolong F, Wang Y, Sun C, Jia L, Mi L, Chen X, Hongtao Cui (2025) : Al-Doped Bismuth Tungsten-Based Composite Coatings for Visible Light Photocatalysis and Long-Term Superhydrophilicity by Laser Deposition. 1–33

Mączka M, Macalik L, Kojima S (2011) Temperature-dependent Raman scattering study of cation-deficient Aurivillius phases: Bi2WO6 and Bi2W2O9. J Phys: Condens Matter 23:405902

Mani S, Bharagava RN (2016) Exposure to Crystal Violet, Its Toxic, Genotoxic and Carcinogenic Effects on Environment and Its Degradation and Detoxification for Environmental Safety. Rev Environ Contam Toxicol 237:71–104

Mirza A, Ahmad R (2020) : An efficient sequestration of toxic crystal violet dye from aqueous solution by Alginate/Pectin nanocomposite: A novel and ecofriendly adsorbent. Groundwater for Sustainable Development 11

Mouni L, Belkhiri L, Bollinger J-C, Bouzaza A, Assadi A, Tirri A, Dahmoune F, Madani K, Remini H (2018) Removal of Methylene Blue from aqueous solutions by adsorption on Kaolin: Kinetic and equilibrium studies. Appl Clay Sci 153:38–45

Mulmi DD, Bhattarai R, Thapa RB, Koju R, Nakarmi ML (2022) : Enhanced photocatalytic degradation of organic pollutants by lysozyme-mediated zinc oxide nanoparticles. Bull Mater Sci 45

Phuruangrat A, Dumrongrojthanath P, Ekthammathat N, Thongtem S, Thongtem T, He T (2014) : Hydrothermal Synthesis, Characterization, and Visible Light-Driven Photocatalytic Properties of Bi2WO6 Nanoplates. Journal of Nanomaterials 2014

Phuruangrat A, Dumrongrojthanath P, Thongtem T, Thongtem S (2017) Hydrothermal synthesis and characterization of visible-light-driven Cl-doped Bi. J Ceram Soc Jpn 125:500–503. %3C sub %3E 2 WO %3C sub %3E 6 nanoplate photocatalyst

Phuruangrat A, Dumrongrojthanath P, Thongtem S, Thongtem T (2018) Hydrothermal synthesis of I-doped Bi2WO6 for using as a visible-light-driven photocatalyst. Mater Lett 224:67–70

Pourhakkak P, Taghizadeh A, Taghizadeh M, Ghaedi M, Haghdoust S (2021) Fundamentals of adsorption technology, Interface Science and Technology. Elsevier, pp 1–70

Rao Q, Zhang Y, Wang R, Zhu R, Fallatah AM, Mersal GAM, Li Y, Tong F, Ibrahim MM, Kuang Y, Yuan B, Yang S (2025) : Advanced selective adsorption of alizarin dye from wastewater using novel nanomagnetic molecularly imprinted polymers. Adv Compos Hybrid Mater 8

Riera-Torres M, Gutiérrez-Bouzán C, Crespi M (2010) Combination of coagulation–flocculation and nanofiltration techniques for dye removal and water reuse in textile effluents. Desalination 252:53–59

Rubio-Clemente A, Chica E, Peñuela GA (2022) : Optimization Method to Determine the Kinetic Rate Constants for the Removal of Benzo[a]pyrene and Anthracene in Water through the Fenton Process. Water 14

Sarfo DK, Kaur A, Marshall DL, O'Mullane AP (2023) Electrochemical degradation and mineralisation of organic dyes in aqueous nitrate solutions. Chemosphere 316:137821

Sayed MA, El-Gamal SMA, Ramadan M, Helmy FM, Mohsen A (2025) Hydrothermal synthesis and structural optimization of Bi(2)O(3)/Bi(2)WO(6) nanocomposites for synergistic photodegradation of Indigo Carmine dye. Sci Rep 15:17260

Singh PK, Yadav JS, Kumar I, Kumar U, Sharma RK (2022) Carpet industry irrigational sources risk assessment: Heavy metal contaminated vegetables and cereal crops in northern India. Toxicol Rep 9:1906–1919

Song XC, Zheng YF, Ma R, Zhang YY, Yin HY (2011) Photocatalytic activities of Mo-doped Bi2WO6 three-dimensional hierarchical microspheres. J Hazard Mater 192:186–191

Tchounwou PB, Yedjou CG, Patlolla AK, Sutton DJ (2012) Heavy metal toxicity and the environment. Exp Suppl 101:133–164

Wach FDHaIE (1995) Raman Spectroscopy of Bismuth Tungstates. J Raman Spectrosc 26:407–412

Wang D, Zhang J, Guo L, Dong X, Shen H, Fu F (2016) Synthesis of nano-porous Bi2WO6 hierarchical microcrystal with selective adsorption for cationic dyes. Mater Res Bull 83:387–395

Wang KS, Wei MC, Peng TH, Li HC, Chao SJ, Hsu TF, Lee HS, Chang SH (2010) Treatment and toxicity evaluation of methylene blue using electrochemical oxidation, fly ash adsorption and combined electrochemical oxidation-fly ash adsorption. J Environ Manage 91:1778–1784

Wang M, Qiao Z, Fang M, Huang Z, Liu Yg, Wu X, Tang C, Tang H, Zhu H (2015) Synthesis of Er-doped Bi2WO6 and enhancement in photocatalytic activity induced by visible light. RSC Adv 5:94887–94894

Wang Y, He J, Peng D, Zhang T, Long F, Zhang X (2018) Enhanced photocatalytic performance of Mg2 + doped Bi2WO6 under simulated visible light irradiation. Ionics 24:2893–2903

William Au SP, Cheryl J, Collie TC, Hsu (1978) Cytogenetic toxicity of gentian violet and crystal violet on mammalian cells in vitro. Mutat Research/Genetic Toxicol 58:269–276

Wu H, Qin M, Zhang D, Chu A, Cao Z, Jia B, Chen P, Yang S, Li X, Qu X, Volinsky AA, Menon M (2016) Synthesis and Characterization of Al-Doped ZnO as a Prospective High Adsorption Material. J Am Ceram Soc 99:3074–3080

Wu J, Zhang W, Tian Z, Zhao Y, Shen Z (2019) : Facile fabrication of Bi2WO6/BiOCl hierarchical structure as adsorbents for methylene blue dye removal. Mater Res Express 6

Xu F, Hirt B, Biswas P, Kline DJ, Yang Y, Wang H, Sehirlioglu A, Zachariah MR (2022) : Superior reactivity of ferroelectric Bi2WO6/aluminum metastable intermolecular composite. Chem Eng Sci 247

Xu X, Ge Y, Li B, Fan F, Wang F (2014) Shape evolution of Eu-doped Bi 2 WO 6 and their photocatalytic properties. Mater Res Bull 59:329–336

Yang L, Peng Y, Qian C, Xing G, He J, Zhao C, Lai B (2021) Enhanced adsorption/photocatalytic removal of Cu(Ⅱ) from wastewater by a novel magnetic chitosan@ bismuth tungstate coated by silver (MCTS-Ag/Bi(2)WO(6)) composite. Chemosphere 263:128120

Zhang F, Sun R, Li R, Song N, Feng L, Zhong S, Zhao Z (2018) Novel La-doped Bi2WO6 photocatalysts with enhanced visible-light photocatalytic activity. J Solgel Sci Technol 86:640–649

Zhang Q, Zhu Y, Yu L, Dong X, Wu Y, Yang B, Xu Q (2022) Preparation of a visible light catalyst of novel Bi2WO6 loaded Ag3PO4 heterojunction and its efficient removal of toluene. Environ Pollutants Bioavailab 34:527–536

Zou J-P, Ma J, Luo J-M, Yu J, He J, Meng Y, Luo Z, Bao S-K, Liu H-L, Luo S-L, Luo X-B, Chen T-C, Suib SL (2015) Fabrication of novel heterostructured few layered WS2-Bi2WO6/Bi3.84W0.16O6.24 composites with enhanced photocatalytic performance. Appl Catal B 179:220–228

List of Figures

Fig. 1

XRD patterns of Al-Bi2WO6

Figure 2 Raman spectrum of Al-Bi2WO6

Figure 3

FTIR spectrum of Al-Bi2WO6

Figure 4

HR-TEM images of Al-Bi2WO6 and d-spacing of respective planes

Figure 5

TEM/EDS analysis of Al-Bi2WO6

Figure 7

Effect of the adsorbent dose on the adsorption of (a) CV dye (C_o = 20 mg/L, t = 20 min, adsorbent dose = 1.5 g/L) (b) MB dye (C_o = 20 mg/L, t = 25 min, adsorbent dose = 2 g/L)

Fig. 8

Effect of contact time on the removal efficiency of (a) CV dye (C_o = 20 mg/L, t = 20 min, adsorbent dose 1.5 g/L) (b) MB dye (Co = 20 mg/L, t = 25 min, adsorbent dose = 2 g/L)

Figure 9

Effect of contact time and initial concentration on the removal of (a) CV dye (t = 50 min, adsorbent dose = 1.5 g/L) and (b) MB dye (t = 60 min, adsorbent dose = 2 g/L)

Fig. 10

Effect of pH solution on the adsorption of (a) CV dye (C_o = 20 mg/L, t = 20 min, adsorbent dose = 1.5 g/L) (b) MB dye (C_o = 20 mg/L, t = 25 min, adsorbent dose = 2 g/L)

Figure 11

PFO kinetic model for adsorption of (a) CV dye and (b) MB dye at different initial concentrations

Figure 12

PSO kinetic model for adsorption of (a) CV dye and (b) MB dye at different initial concentrations

Figure 13

IPD kinetic model for adsorption of (a) CV dye and (b) MB dye at different initial concentrations

Figure 14

Langmuir isotherm model of (a) CV dye and (b) MB dye adsorption

Figure 15

Freundlich isotherm model of (a) CV dye and (b) MB dye adsorption

Figure 16

Effect of temperature on removal of (a) CV dye [C_o = 20 mg/L, t = 20 min, adsorbent dose = 0.075 g (per 50 mL of dye solution)] and (b) MB dye [C_o = 20 mg/L, t = 25 min, adsorbent dose = 0.1 g (per 50 mL of dye)

Fig. 17

Van’t Hoff plot to obtain thermodynamics parameter of (a) CV adsorption and (b) MB adsorption of dyes

Fig. 1

Fig. 1: XRD patterns of Al-Bi2WO6.

Fig. 2

Raman spectrum of Al-Bi2WO6.

Fig. 3

FTIR spectrum of Al-Bi2WO6.

Fig. 4

HR-TEM images of Al-Bi2WO6 and d-spacing of respective planes.

Fig. 5

TEM/EDS analysis of Al-Bi2WO6

Fig. 6

N2 adsorption-desorption isotherms of the Al-Bi2WO6

Fig. 7

Effect of the adsorbent dose on the adsorption of (a) CV dye (C_o = 20 mg/L, t = 20 min, adsorbent dose = 1.5 g/L) (b) MB dye (C_o = 20 mg/L, t = 25 min, adsorbent dose = 2 g/L).

Fig. 8

Effect of contact time on the removal efficiency of (a) CV dye (C_o = 20 mg/L, t = 20 min, adsorbent dose = 1.5 g/L) (b) MB dye (C_o = 20 mg/L, t = 25 min, adsorbent dose = 2 g/L).

Fig. 9

Effect of contact time and initial concentration on the removal of (a) CV dye (t = 50 min, adsorbent dose = 1.5 g/L) and (b) MB dye (t = 60 min, adsorbent dose = 2 g/L).

Fig. 10

Effecct of pH solution on the adsorption of (a) CV dye (C_o = 20 mg/L, t = 20 min, adsorbent dose = 1.5 g/L) (b) MB dye (C_o = 20 mg/L, t = 25 min, adsorbent dose = 2 g/L).

Fig. 11

PFO kinetic model for adsorption of (a) CV dye and (b) MB dye at different initial concentrations

Fig. 12

PSO kinetic model for adsorption of (a) CV dye and (b) MB dye at different initial concentrations.

Fig. 13

IPD kinetic model for adsorption of (a) CV dye and (b) MB dye at different initial concentrations.

Fig. 14

Langmuir isotherm model of (a) CV dye and (b) MB dye adsorption.

Fig. 15

Freundlich isotherm model of (a) CV dye and (b) MB dye adsorption.

Fig. 16

Fig. 17

Van’t Hoff plot to obtain thermodynamics parameter of (a) CV adsorption and (b) MB adsorption of dyes.

List of Tables

Kinetic variables for the adsorption of CV and MB dyes

Langmuir and Freundlich adsorption isotherm constant for adsorption of CV and MB dyes.

Thermodynamic influencing factors for adsorption of CV and MB dyes