Sawdust-activated biochar impregnated with a spinel structure effectively removes textile dye wastewater

Due to rising dye-laden industrial effluents, aquatic ecosystems and human health need efficient, long-term, and cost-effective treatment options. The nanocomposite material produced from Aluminium Ferrite (AlFe₂O₄) and Sawdust (SD) Biomass neutralised synthetic dyes in water. Raw sawdust was mixed with catalytic and magnetic spinel-type ferrite AlFe₂O₄ to maximise adsorption and photocatalytic-like breakdown. The Biochar@AlFe₂O₄ nanocomposite was developed by biochar in an acidic medium and Spinel Ferrite. Characterisation by TGA, XRD, and FTIR analysis indicated the formation of AlFe₂O₄ NPs to determine its structural and optical properties. A weak band at ~ 3739.9 cm⁻¹ is attributed to the O–H stretching vibrations of unbound hydroxyl groups, possibly from residual surface –OH functionalities or moisture in the nanocomposite. Al-O bending at ~ 440 - 446 cm-1 confirms metallic bonding at structural lattice tetrahedral sites. The hybrid system enhances dye removal efficiency through two mechanisms: lignocellulosic functional group adsorption and catalytic oxidation via hydroxyl radical production. Biochar@AlFe₂O₄ with a band gap energy of ~ 3.00 eV was examined by the DRS method. Magnetic susceptibility shows saturation magnetisation (Ms) = ~ 24.59 emu/g, a remanent magnetisation (Mr) = ~ 3.64 emu/g, and a coercivity (Hc) = ~ 26.78 Oe, which is sufficient to recover the catalyst for reuse. The impacts of operational parameters such as pH, contact time, dye concentration, and catalyst dose were thoroughly studied.

Furthermore, the composite's reusability and metal leaching potential were evaluated to determine its environmental sustainability. The experimental and predicted adsorption-photocatalytic with H₂O_2, the possibility for merging low-cost biomass with underexplored ferrite materials such as AlFe₂O₄ for AOPs, paving the way for circular, green remediation technologies. It occurred with a high degradation rate at ~ 100 % in neutral conditions, in ~ 90 mins duration. It is recyclable and maintains structural and photocatalytic stability over 4 consecutive stages.

Introduction

Textile wastewater is a highly polluting form of industrial waste after being discharged through pipes to damping into the sea or lakes, which adversely affects the human environment. Previously, one reported that the number of industries in India produces a million metric tons of wastewater every year. Last year's data showed that the majors had high COD, BOD, and DO in wastewater and organic pollutants produced by textile industries (Mahdizadeh et al., 2020). Although CPCB (Central Pollution Control Board) authorities have reported that the 15% water consumption rate should be decreased in the textile sectors in India.  It is a great initiative for minimising the pollutants in Textile units by CPCB.  

One of the most prominent waste materials abundantly used in renewable energy sources to produce Biofuel, Biochar, oils, and gases, i.e., Biomass. It is also reliable for the efficient use of waste into a product. Further utilisation is accompanied by a source of degradation of toxic molecules in wastewater. RhB is a cationic dye that is frequently used in the textile sector, for dying cotton, wool, silk and acrylic. This dye has been reported to impair the photosynthetic activity in plants. RhB is carcinogenic, harmful to humans, and it is imperative to treat RhB dye from textile effluents before dumping it into freshwater ecosystems.   

Various conventional physical and chemical methods are employed to treat coloured wastewater, including RO (Reverse Osmosis), Sedimentation, Ultrafiltration, electrochemical adsorption, catalysis, coagulation, ion exchange, electrolysis, and photo-oxidation. These methods are extensively employed in the study of textile wastewater treatment. The efficiency of adsorption in the decolourisation of textile effluent was attributed to its friendly operation, design simplicity, and insensitivity to toxicity, which were among the technologies that were frequently employed. Commercially activated carbon, chemically modified AC, and AC prepared physically from pyrolysis are all examples of conventional adsorbents (Amin et al., 2017).

As one of the most promising and highly considerable attention owing to its ability to tackle emerging contaminants effectively. Different Reactive Oxygen Species (ROS), sulphite radical, Hydroxyl radical, carbonate radical and some other functions are added based on their reaction in the presence of visible/UV ranges (Haider Kazmi et al., 2025). Among different Advance Oxidation Processes (AOPs)s, the hybrid methods are best suited to produce Hydroxyl radicals in the presence of a suitable light source, after calibration of the dye. OH, radical hampers the complete mineralisation of the contaminants. Further, huge energy consumption has limited the use of AOPs for industrial applications.

Sawdust (SD) may play an important role in tackling this complex challenge, offering a sustainable solution for environmental remediation and resource recovery. SD has the potential to enhance the current efficiency and promote the complete mineralisation of contaminants by inhibiting side reactions such as OER. Nevertheless, the brief cycling lifespan suggests that SD is of high mechanical and chemical stability, which may result in rapid degradation, a loss of structure, or reduced performance after repeated use. SD is suitable for long-term use, and it is possible to accomplish significant degradation results (Fito & Nkambule, 2023). Reusing and recycling SD and converting it into biochar can provide low-cost and efficient sorbents for the remediation of a range of environmental contamination(Ahmadi et al., 2025). For instance, biochar produced from wooden waste is considered a low-cost sorbent and has exhibited excellent results in heavy metals removal from wastewater. SD is abundant in various parts of Bhopal in Madhya Pradesh, India. It consists of SD, which contains Hemicellulose, Cellulose, Lignin in higher proportions. Therefore, it was hypothesised that pyrolyzed the waste biomass may produce efficient SD based on proximate and ultimate analysis. Thus, the purpose of this study was to explore the potential applicability of preparing biochar of an agro-industrial waste obtained from SD and using it as a bio-sorbent materials for RhB dye removal from synthesis wastewater treatment from aqueous solutions.

In this field, a variety of chemicals have been developed and utilised, one of which is the production of hydroxide radicals using photocatalysis to remove contaminants from wastewater. Fenton reagent, also known as Photo-Fenton, can be adjusted in comparison to photocatalysts to evaluate degradation efficiency while maintaining a minimal contact range for the RhB dye. Because the molecules are rigid and difficult to break bonds with adsorption and photocatalytic methods, sawdust Biochar is utilised in parallel for degradation rather than the spinel catalyst. Both have a high surface area to maximise the outcomes. The evaluation was done based on optimisation, and both spinels synthesised via sol-gel synthesis are attractively adopted on the surface of biochar. The minimum dosage required to function as an active catalyst for dye molecule degradation in the presence of a visible light source. Both were generated in 1:2 and 1:4 catalytic material ratios for the photocatalytic over-adsorption method.

M⁺ + H₂O₂→ M⁽ⁿ⁺¹⁾ + + HO• + HO⁻

Table 1: Literature Survey on Waste Materials Enhancing the Pyrolysis Process

S.No.	Materials	Method	Parameters	Applications	Major points	References
1	Sawdust	Pyrolysis	Heating rate: 3.3, 6.7, 10, 20˚C/min., Time: 60-90 min., Temperature: 500, 650˚C	Pore formation, pyrolysis kinetics	Impregnated with H3PO4	(Xie et al., 2025)
2	Municipal Solid Waste and Sawdust	Co-Pyrolysis	Heating rate: 10-40˚C, Temperature: 500-600˚C, blending ratio: 0.12-0.3	MSW-SD optimization processes for economical co-pyrolysis	Isothermal modelling with machine learning, parameters optimization	(Najar & Rasool, 2025)
3	Sawdust-metal oxide catalyst	Pyrolysis	Heating rate: 35˚C/min. Temperature: 550˚C	Biocrude from Biomass	Vapor phase upgrading biomass over a catalyst to enhance bio-yield	(Singh et al., 2025)
4	Fir wood sawdust, olive stone and sewage sludge	Pyrolysis	Temperature: 400˚C to 800˚C	Pyrolysis with molten carbonate salts verify the product biooil for hybrid thermochemical biological treatment due to reduction of water-soluble components.	Li2CO3-Na2CO3-K2CO3 salt used to analyse and with generation of pyrolysis product	(Ahmadi et al., 2025)
5	NaCl-FeCl  2, NaCl-ZnCl  2, and NaCl-MgCl  2, pine sawdust	Catalytic pyrolysis	Temperature: 600˚C	HZSM-5/cordierite monolithic catalysts were prepared, Binary molten chlorides served as key intermediates to produce aromatics with high selectivity in a single step, Production of aromatics, biomass into aromatics.	Van Krevelen Diagram indicated that aromatics form the bio-oil	(Wang et al., 2025)
6	Banana Peel Biochar-Ti/GP	Pyrolysis	Time: 2h, Heating Rate: 10˚C/min., N2 gas environment, Temperature: 600˚C	Removal of carcinogenic methyl violet 2B dye using with BPB-Ti/GP)	10 ppm conc. Of dye, 0.2 M Na2SO4, pH 9.0 in electrolysis.	(Haider Kazmi et al., 2025)
7	Palm empty fruit bunch and Rubber wood sawdust	Co-Pyrolysis	Temperature: 750˚C,800˚C, 850˚C, N2 atmosphere	Optimization on co-pyrolysis of oil pam empty fruit bunch and Rubber sawdust systematically on Response surface methodology, Aspen Plus simulations with experimental data.	Used Aspen Plus Simulations to validate the experimental data and confirmed the process.	(Promraksa et al., 2025)
8	Wood Sawdust	Co-Pyrolysis	Heating Rate: 10˚C/min., N2 atmosphere, Temperature: 500˚C	Plastic loading on Biochar, increased hardness and reduced the roughness of the char, for good in energy storage, and carbon-based materials.	Plastic incorporation with biochar which is reduce surface area, increases potential to use for energy storage and other industrial applications	(Mishra, 2025)
9	Sawdust	Pyrolysis	Heating rate 3˚C/min, N2 atmosphere, Temperature 400˚C	Pyrolysis for Biochar Synthesis Optimization for Catalyst Loading	To improve RhB dye organic pollutant surface area targeted	This work

 

Table 2: Literature Survey on Pollutant Degradation Using Hybrid-AOPs

Hybrid AOP	Process Description	Materials Used	Target Pollutants	Performance	Reference
Photocatalysis + Ozonation	Combines photocatalytic oxidation with ozonation to generate hydroxyl radicals, enhancing pollutant degradation.	Ag/ZnO nanocomposites	Phenol	Significant enhancement in photocatalytic ozonation efficiency.	(Están García et al., 2025)
Hydrodynamic Cavitation + Photocatalysis	Utilizes cavitation-induced microbubbles and photocatalysis for effective degradation.	ZnO/ZnFe₂O₄ nanocomposites	Carbamazepine	Achieved 98% degradation efficiency.	(Bhatti & Parikh, 2022)
Photo-Fenton + Photocatalysis	Integrates photo-Fenton reactions with photocatalysis under solar irradiation to enhance hydroxyl radical production.	TiO₂, Fe³⁺ ions	Malachite Green dye	Complete degradation achieved.	(Feiqiang et al., 2018)
Electrocatalysis + Ozonation	Combines electrochemical oxidation with ozonation to improve mineralization of pollutants.	Ti/PtIr electrodes	Industrial effluents	Achieved 100% COD removal.	(Pasciucco et al., 2025)
Ultrasound + Photocatalysis	Uses ultrasonic waves to enhance photocatalytic degradation through cavitation effects.	TiO₂ nanoparticles	Various organic pollutants	Enhanced degradation efficiency compared to individual processes.	(Wolski et al., 2024)
UV/H₂O₂ + Adsorption	Combines UV-induced hydrogen peroxide oxidation with activated carbon adsorption for pollutant removal.	H₂O₂, Activated Carbon	Pharmaceutical effluents	COD removal increased from 75-88% (AOP alone) to up to 93% with adsorption.	(Martínez et al., 2013)
Fenton + Electro-Fenton	Integrates Fenton's reagent with electrochemical regeneration of Fe²⁺ ions to sustain hydroxyl radical production.	Fe²⁺/Fe³⁺ ions, Electrodes	Industrial wastewater	Achieved high degradation efficiencies with continuous Fe²⁺ regeneration.	(Tripathy & Prakash, 2025)
Photocatalysis + Membrane Filtration	Combines photocatalytic degradation with membrane separation to remove degraded pollutants.	TiO₂-coated membranes	Dye wastewater	Achieved 100% degradation with effective separation.	(Ambegaonkar et al., 2025)

 

Table 3: Emerging Nanomaterial-Based Techniques for Pollutant Degradation

Nanomaterials/ Adsorbents/Catalysis	Application/Target Pollutant	Mechanisms/Key Findings	References
Ultrasound-Based Hybrid Technology (AOP)s	Organic Pollutant	Focus on Efficiency in organic pollutant degradation in aqueous environment conditions.	(Choi, 2019)
Doped-TiO2 Photocatalysis Hybrid-AOPs	Non-Biodegradable in Wastewater	Industrial or synthetic effluents. Doped or Co-Doped Photocatalysis. Study on Parameters affecting photocatalysis. Addition of Biochar on photocatalysis.	(Bhatti & Parikh, 2022)
PEC fuel Cells (C3N5, CuO/Cu2O)	Antibiotic Contaminants Degradation	High reactive system in PEC-PMS system, Visible light (120 mins.). This approach is used for both water cleanup and an energy reuse system.	(Wei et al., 2025)
Heteroatom-doped Co-Carbon	Organic Pollutant	Rich pore structure, Surface chemical functional group, Graphite Defect structure, 100 ppm conc. MB sol., Small-scale fixed-bed reactor is used.	(Li et al., 2024)
TiO2/bentonite	Propanenolal removal from wastewater (Pharmaceutical contamination)	Solvothermal treatment, photocatalytic activity, 99% removal in 60 mins.	(Rosa et al., 2025)
ZnO nanostructures/Moringa oleifera leaves extract	Erythrosine dye removal from wastewater	Antibacterial activity, Natural sunlight, pH, cycling activity, catalyst dose, diff. conc., t E. coli and Bacillus subtilis bacteria studies,	(Parven et al., 2025)
Alg-Fe3O4@GO	Cationic dye wastewater	Adsorption and photocatalytic, MB % MG dye wastewater, Surface area 317.8 m2/g, t=120 mins in the Visible light range. Degradation of 85% of MB and 78% of MG	(Mubarak et al., 2025)
Fe-Mn/ZrO2	cinnamic acid (CA), benzoic acid (BA), and catechol (C) removal	Focused on ROS (reactive oxygen species) generation, the best degradation results are with CA.	(Loffredo et al., 2023)
Fe-Cu@carbon	bisphenol A	AOPs used, BPA degradation in 10 mins of reaction, 5 generation cycles, good magnetic properties.	(Están García et al., 2025)
CPAN@ZnO-Ag	Congo Red removal	98.4% removal efficiency in 90 mins. Photocatalytic work on actual wastewater and achieving use of solar energy and conservation.	(Song et al., 2025)
CuS0.78Se0.22	Cu (II)-EDTA degradation	A good adsorption capacity, an electrooxidation-capacitive deionisation (EO-CDI) system is proposed.	(Shen et al., 2025)
Nitrogen-doped open hollow carbon structure (Co-NOHC)	Organic pollutant (RhB degradation)	Outstanding peroxy-monosulfate activation activity and Fenton-like performance in oxidation by rhodamine B degradation.	(Fu et al., 2025)
Electrochemical Advanced Oxidation Processes (EAOPs	Phenol oxidation degradation	The graphite-based boron-doped diamond (BDD) coated Electrodes were prepared by pre-depositing a refractory metal (Mo/Nb/Ti/Ta) interlayer on a graphite substrate, with an electrochemical window (from −1.34 V to 2.61 V).	(Gong et al., 2025)
FeMoO4-based sludge-derived catalysts (MoS2-SDC-n)	Wastewater sludge is used to remove organic contaminants in wastewater	The hydrothermal process is used to prepare the catalyst, reactive sites for promoting SO4•− generation, and a new sludge-based catalyst design.	(Yao et al. 2025)
rGO@MoS2 nanocomposites	degradation of chloramphenicol from contaminated water	Contaminated reduction under UV irradiation, hydrothermally prepared catalysis, 86 % within 180 min at neutral pH.	(Raj et al., 2025)
Biochar@AlFe2O4	RhB Dye Synthetic wastewater	Under Visible irradiation with H2O2, contaminated reduction was achieved by using a Sol-gel produced catalyst in a 1:1 ratio with sawdust biochar, degrading synthetic 20 ppm RhB dye wastewater by 98.8% in 90 minutes at neutral pH.	This work

 

Materials and Synthesis Methods

Material required for the synthesis of a nano-composite 

Aluminium Nitrate (AR-grade chemical with 98% purity) (Al(NO₃)₃.9H₂O), Ferric Nitrate (99%)(Fe(NO₃)₃.9H₂O), Citric acid (C₆H₈O₇), Ethylene glycol(C₂H₆O₂), and Millipore water (H₂O), utilised for Spinel synthesis, were procured from Loba Chemie Pvt. Ltd, ICHEM Laboratories, Kochi, Sigma Aldrich, Rhodamine B (C₂₈H₃₁ClN₂O₃) AR grade dye were used in this study.

Synthesis of Bio-Char experiment set-up

In this paper, waste sawdust (biomass), is treated with 0.1M sulfuric acid due to oxygen-containing functional groups (-COOH, -OH, -SO₃H) on the biomass surface to increase the chemical reactivity of biochar. It hydrolyses hemicellulose and lignin, enhancing porosity in the sawdust structure. Acid treatment at 60˚C for 18 hours involved mixing sawdust and H₂SO₄ in a 1:4 mass ratio after successful pre-treatment, then adjusting the pH with Millipore water to neutral. Acid treatment causes chemical and structural modification to the lignocellulosic biomass, and it hydrolyses cellulose, leading to glucose release, and even dehydration into furfural in a strong range. It is more resistant but partially solubilised or degraded, depending on acid strength and enhances the accessibility of cellulose fibres.

The sawdust (SD) wooden biomass used in this study, after getting acid treatment utilised by sulphuric acid (H₂SO₄). The proximate and ultimate analyses of the sawdust biomass used in this research are discussed in Table 4. (CHNSO Analyser), it is shown that mass losses occur due to the solubilization of organic components and the addition of -SO₃H groups, which increases the acidity of the treated materials, making them useful in catalysis and adsorption.

Pyrolysis of Biomass for Biochar

The precursor was dried at 80˚C for 24 hours, yielding a low percentage of moisture content in biomass. Then, proceed to pyrolysis at 400˚C, rate 3˚C/min. for 130 minutes under a N2-controlled environment. The Biochar produced in the tube furnace was cleaned with Millipore water until it had a neutral pH. To produce the biochar, it was dried in an oven at 80˚C for 12 hours.

 

Synthesis of nanocomposites material

Precursors are procured from Merck. Implementation for spinel preparation by the sol-gel synthesis method, because it is easy to generate spinel NPs with appropriate handling. Aluminium Nitrate Nonahydrate and Iron Nitrate Nonahydrate (1:2), Citric acid (1M), Ammonia solution are utilised for the synthesis of NPs via the sol-gel procedure. Initially, all precursors are mixed in a small container using a magnetic stirrer and a hot plate at 80˚C for 2 hr with 6 mL of Ammonia solution for a neutral pH. Then, after the sol is produced, enough water is added for centrifugation at 12,000 rpm for 30 minutes to separate the sol, then dried for 6 hr at 80˚C. Then, caline at 700˚C, to make it enough with a C/B ratio of (1:1) of Biochar and spinel to make a composite of the materials. The C/B ratio was selected to investigate the effects of low and high catalyst loadings on product yield, composition and f ormation based on prior studies that employed varied ratios (Singh et al., 2025). Sonication was used for 30 mins with the mixture then further heating at 350˚C for 2 hours. Then, Composite phase was confirmed by XRD. 

Characterization results

TGA analysis (model: TGA 8000/ Pyris 1 TGA) was performed to analyse the pyrolysis process of sawdust from 30˚C to 900˚C under N₂ atmosphere, with heating rates of 10˚C/min. Furthermore, to acquire the carbon formation condition and produce biochar. In the TGA study, SD@AT (Acid Treated) displays different results from Raw Sawdust (RSD). Weight loss at 300˚C abruptly reduced till 400˚C, and materials can become a high content of carbon by pyrolysis of sawdus

TGA-DTG (TGA 8000 Pyris 1) analysis on RSD and SD@AT utilised thermal decomposition processes of SD can be categorised into four different stages, with an overlap between different stages of temperature from 30˚C to 900˚C. The curve line shows the RSD weight losses at 100˚C due to the loss of physically adsorbed water and moisture, but after acid treatment, extreme deformation occurs at the same temperature, further decreasing at 400˚C when the moisture content is similar at 100˚C. From 200 to 400˚C, major degradation of weight and decomposition of hemicellulose, cellulose, and some lignin, which is called peak thermal degradation. Both materials can become carbonised when the temperature rises to < 400˚C, slow degradation of lignin and fixed carbon, and retain slightly more mass, higher thermal stability or higher fixed carbon content predicted to be achieved with post-treatment. Figure (a) shows that biochar can be produced from raw sawdust at temperatures ranging from 300 to 500˚C. A high carbon content leads to production at temperatures above 500˚C.

Table: "Comparative analysis of TGA-DTG data reveals predictive insights into the thermal degradation behaviour of RSD and SD@AT."

Parameters	RSD	SD@AT
Initial Degradation	100-150 ˚C	100-150 ˚C
Major Degradation Temperature	250-400 ˚C	250-400 ˚C
Final Residue (%)	17-18%	20-21%
Thermal Stability	Lower	Higher

 

SD@AT  shows greater thermal stability and higher residual mass, possibly due to acid treatment increasing fixed carbon (FC) or removing volatile organics. It is suitable for low-temperature applications or as an untreated biosorbent, and good adsorption, carbonaceous materials synthesis or composite fillers due to enhanced thermal stability. Similarly, in the DTG, derivative curves are plotted to show the mass losses at 380-400˚C which means stable for Biochar production using SD in the absence of oxygen. (a) TGA-DTG analysis for Sawdust, and Spinel NMs (b) FTIR analysis for biomass-derived nanocomposite and materials studies, (c) XRD analysis for Biochar@AlFe₂O₄ Nanocomposite and materials, (d) Band Gap Analysis for Nanocomposite

The average crystalline size determined by XRD data is 24.48 nm, indicating strong results for nanocrystalline materials for Biochar@AlFe₂O_4, which is good for the photocatalysis-adsorption process. The peaks are (220), (311), (400), (511), and (440), confirming that the spinel phase occurred to form a Cubic pair (Fd3m), as confirmed by JCPDS no. 01-089-7408 of AlFe₂O₄, from the support, with a small crystalline size and thermal gradients during calcination. It shows that the materials were synthesised with well-defined crystals and a large surface area for catalysis or adsorption under good magnetic and structural properties. The peak data determined by XRD are 30.4, 35.8, 43.6, 53.9, 57.5, 57.94, and 63.3˚. The highest peak value was 35.8˚, corresponding to specific planes of different crystalline phases, depending on sample composition and X-ray source (assuming Cu Kα, λ=1.5406 Å). Although the biochar amorphous peaks determine the confirmation peaks of carbon present in it and Spinel takeover onto the biochar surface. (001) and (022) show the maximum significance of the carbon C60 structure over the catalyst surface. It is predicted that composite formation will be completed by a simple sol-gel synthesis method. 

In the FTIR understanding of sawdust or lignocellulosic biomass, the absorption band observed at approximately 1020 cm⁻¹ is attributed to the C–O stretching vibrations predominantly arising from cellulose and hemicellulose components. This region represents the characteristic fingerprint of polysaccharide structures, and its intensity is sensitive to chemical modifications such as carbonisation or surface functionalization. The C–O bonds present in cellulose and hemicellulose are primarily associated with alcoholic and ether functional groups. A distinct peak observed at ~1512.9 cm⁻¹ corresponds to the aromatic C=C skeletal stretching vibrations within the lignin matrix, a complex phenolic biopolymer naturally present in wood. This band is indicative of the aromatic structure in lignin, typically exhibiting strong and sharp intensity in raw biomass. However, upon thermal treatment (e.g., pyrolysis or carbonisation), this peak often diminishes in intensity or undergoes a spectral shift, signifying the depolymerisation or breakdown of lignin. The presence of a weak band near ~2308 cm⁻¹ is considered atypical for native lignocellulosic materials. This peak is generally attributed to asymmetric stretching vibrations of carbon dioxide (CO₂), likely introduced through atmospheric absorption or residual gases trapped during sample preparation. A broad but weak band around ~3739.9 cm⁻¹ is assigned to the O–H stretching vibrations of free (non-hydrogen bonded) hydroxyl groups, which may originate from residual surface –OH functionalities or moisture retained in the sample. This region can also reflect contributions from unbound alcohol groups or physically adsorbed water, especially in unmodified or slightly treated biomass.

Effects of solution pH, Contact time, Dye concentrations, and Catalyst dose

The adsorption experiments were performed in triplicate to facilitate statistical analysis and minimise experimental error. The required amount of RhB was suspended in 100 mL of Milipore water solution. In a Borosilicate glass, Volume = 250 ml, containing a specified amount of 0.1 g Biochar@AlFe₂O₄ dosage for adsorbents. Band Gap estimated optical band gap for Biochar@AlFe₂O₄ composite, assuming direct band gap, which is useful for the application of photocatalysis, solar-driven dye degradation, as we considered. According to our Tauc plot, the direct band gap is ~3.00 eV, which is optically active in the visible region, and with the application of photocatalysis, either could be good to achieve RhB dye degradation.

Image showing RhB dye degradation at 20, 60, and 100 ppm concentration in the presence of a UVC light source with 0.3 g of catalyst dosage.

Influence of constant time

To design the photoreactor, use Pyrex glass with a volume of 250 ml. Then, provide the light source to employ a visible light source with a wavelength range of 400-700 nm that is considered for this setup. To catalyse the dosage on load with deviation from 0.1-1.2 g of the catalyst into the photoreactor. Add 100 mL of Millipore water with 9.79 mmol H₂O₂ to the photoreactor. This photocatalytic process using Biochar@AlFe₂O₄ involves harnessing light energy at the visible range to possibly degrade organic contaminants in wastewater simultaneously.

AlFe₂O₄ is a visible light active spinel ferrite semiconductor with a narrow bandgap (3.00 eV), which might occur, allowing it to absorb light when light hits AlFe₂O₄ Spinel Nanomaterials. The electron (e^-) goes to the conduction band, and the hole pairs are generated in the valence band. Conduction band Electrons (e^-) (Rashid Ahmed & Kayani, 2024). Biochar decreases electron and hole recombination by acting as an electron sink. It also improves dye adsorption by promoting H₂O₂  activation and providing a surface for improved AlFe₂O₄ dispersion. (Cheng et al., 2021).

AlFe₂O₄ + hv – e^- + h⁺ …………………………………………………………...(1)

e- + H₂O₂  → •OH + OH^-    ………………………………………………………(2)

h⁺ + H₂O → •OH+ H⁺ ……………………………………………………………(3)

•OH + RhB → intermediates → CO₂ + H₂O + by products ……………………..(4)

RhB + hv + Biochar@AlFe₂O₄  (Catalyst) + H₂O₂ → CO₂ + H₂O + degradation products (overall reaction) …………………………………………………………………(5)

H⁺ organic contaminants oxidation products (e.g., CO₂, H₂O). Holes oxidise organic matter in wastewater, aiding degradation. The results are based on material results, and the ~3.00 eV energy is estimated. Spinel ferrite nanomaterials are excellent adsorbents since they possess a high surface area and are highly porous, which means a greater number of active sites. These sites interact with the containers and attach them, enabling easy removal using magnetic separation.

Determination of dye degradation in alkaline or acidic environmental conditions

pH procedure with different variations, set up with varying concentrations over time. The pH of the RhB concentration solution was adjusted using 0.1M NaOH and 0.1M HCl. The mixed nano composite adsorbent and the Dye solution were stirred for a specific time duration (NEUATION iSTIR HP550 Prime). Initially, dye 100 ml volumes were set at a ~3.0 pH range, which gives a low degradation rate in an acidic environment. When simultaneously rising pH, the RhB dye degrades with its efficiency and rate of colour change of color fade at initial contact time. The concentration of 20 ppm is optimised with different time intervals (10 to 90 minutes) as per the analysis of RhB dye rates at pH 7.0. At neutral pH, in visible light, light penetrates the surface of the solution, which produces electrons and holes that are generated from the valence band to the conduction band and produce •OH (Hydroxyl) radicals. Al⁽³⁺⁾ reacts with water to produce hydroxyl radicals and combines with OH^- ions to degrade the rigid bond of RhB dye in water. The optimisation has occurred at different pH conditions, but the best degradation happened at pH 7. When increasing pH, the degradation efficiency will reduce with respect to contact time.

The degradation efficiency of Rodamine B dye (RhB dye) by the catalyst Biochar@AlFe₂O₄ composite materials' surface becomes positively charged due to an excess of H⁺ ions. RhB is the cationic dye, so it gets electrostatic repulsion that occurs between RhB and the catalyst, with less Hydroxyl (∙OH) formation of fewer OH- ions available in solution to generate radicals. So, it is a weak interaction and poor radical formation with low degradation rates. When pH is 7 (neutral), surface charge occurs less repulsion between RhB and the catalyst. Optimal generation of (∙OH) and better electron transfer. RhB degradation via the photocatalytic process dominates in an ideal environment, resulting in a higher degradation rate. A catalyst that becomes negatively charged might attract RhB, but too many OH^- ions can scavenge holes (h⁺) generation or quench radicals. Degradation slows if radical recombination increases or dye structure resists breakdown. It is better than acidic but not as efficient as neutral degradation. 

The concentration of RhB was determined using a UV-vis spectrophotometer (Lambda 650S, UV-Vis spectrophotometer, PerkinElmer, SA) at the fixed wavelength of 523 nm, which results in the highest peaks. After the adsorption treatment, the supernatant liquid was analysed to verify the reduction of RhB concentration. The removal efficiency of correlation estimation is:

%R is the removal percentage of the RhB dye concentration from solution using photocatalytic-adsoption phenomen from solution using adsoption, qt is the amount of RhB attrached per unit mass of the nanocomposite, C₀ is the initial concentration of RhB at the time (t) and V is the volume of the aqueous solution, and M(g) is the dry mass of adsorbent.

Table 7: Summarise pH variation of RhB over Biochar@AlFe₂O₄ composite

pH	Catalyst Surface	Interaction with RhB	∙OH formation	Degradation
3	Positively Charged	Repulsion	Low	Low
7	Neutral	Ideal Interaction	Optimal	High
11	Negatively Charged	Attraction	Medium	Medium

 

The coefficients of determination over catalyst dosage, Concentrations over time, and acidic and alkaline environmental conditions with RhB dye. The catalyst shows linear fitting data at higher dosage, while at low pH, = ~3  The scale is enough for a linear regression fitting curve. 0.3 g is the optimal catalyst range in the set of all the dye concentrations. R² = ~ 0.97-0.99 is more acceptable for a linear fitting curve.                   

Effect of Adsorbent Dosage on Photocatalytic-Adsorption Activity

Biochar@AlFe₂O₄ was utilised in the same volume over a catalyst dosage of 0.1 to 1.0 g of RhB dye, which was a total volume of 100 ml, in the presence of Visible light in the photocatalytic reactor.  The effect of the catalyst dose on the RhB dye experimentation structure followed (Nasir et al., 2025). The degradation reached ~ 100 % in 90 minutes. However, as the dosage has already been optimised for greater RhB dye degradation results, no additional dosage increases are necessary. Increased light surface penetration and less light reaching the catalyst surface could result from higher concentrations, which would also prevent the production of reactive species. Additional overdosing may increase the recombination of photogenerated electron-hole pairs, decrease the catalyst's capacity for redox reactions, and result in a high cost. Increased concentration, which limits the reaction rate because of mass transfer limitation, prevents RhB dye molecules from diffusing to the catalyst surface.

Recyclability

To determine the stability of the Biochar@AlFe₂O₄ nanocomposite, it was subjected to several cycles of RhB dye photodegradation in four recycling runs in a row. In the remaining runs, AlFe₂O₄ has a greater impact on the high surface. The photocatalyst's outstanding reusability, achieving an efficiency of 85.52% on the first cycle and just slightly declining to 71.2% and 64.70%, 53.99% on the subsequent cycles, respectively. This process results in a decline in RhB degradation efficiency over time (Bouchenak et al., 2025) was initially determined to be ~ 20 ppm to balance adequate and surface interactions and achieve effective photocatalytic degradation under the experimental conditions. Therefore, for environmental remediation applications, the nano-composite exhibits good stability and recoverability of these nanoparticles.

The DLS predicted analysis might be from the data volume-weighted size distribution of RhB dye before and after degrading treatment demonstrates a distinct alteration in the particle properties of the system. The sample initially displayed a narrow and strongly defined distribution centred at approximately 350-400 nm, indicative of a substantially monodispersed population of dye-associated aggregates. Following degradation, the distribution significantly expanded, with the principal peak diminished in amplitude and a notable tail continuing toward greater hydrodynamic diameters up to 1 µm. The quantitative study of the plotted distributions indicates that volume-weighted mean diameters escalated from 370 nm in the initial sample to 409 nm in the deteriorated sample, while the estimated polydispersity metric went from 0.05 to 0.17, signifying a notable rise in size heterogeneity. The degradation process not only breaks dye molecules but also facilitates the creation of bigger aggregates of degradation intermediates, dye-catalyst contacts, or composite-associated complexes within the reaction system. The manifestation of this high-diameter tail, although with partial dye degradation, illustrates the sensitivity of volume-weight. DLS quantifies even minor populations of substantial aggregates.

The magnetic behaviour of Biochar@AlFe₂O₄ nanocomposite, which is a ferromagnetic material with the presence of Fe²⁺ ions in the material. The magnetism hysteresis loop demonstrates the character of Al-Fe NCMs. The VSM analysis demonstrated that the Al–Fe nanocomposite materials exhibited soft magnetic properties, with a saturation magnetisation (Ms) = ~ 24.59 emu/g, a remanent magnetisation (Mr) = ~ 3.64 emu/g, and a coercivity (Hc) = ~ 26.78 Oe. The elevated Ms and diminished Hc indicate that the material exhibits significant magnetic responsiveness, facilitating the separation of the adsorbent from the treated solution post-dye adsorption. The magnetic recoverability enhances the utility and reusability of Al–Fe NCMs for the treatment of dye wastewater.

Conclusion

The application of conventional wastewater for the removal of RhB dye from Textile wastewater was measured in various factors. The Biochar@AlFe₂O₄ nanocomposite was effective in degrading RhB dye in a visible light environment. The sol-gel precipitation synthesised this nanocomposite. Ferric oxide functional groups were present, as validated by FTIR data, and XRD confirmed the pure phase of aluminium iron oxide. The maximum adsorption of RhB with a band gap estimated across ~ 3.00 eV as per the Tauc plot, after the dye degradation efficiency was found to be 100 % at the optimum condition of photocatalytic reaction with 0.4 deviation dosage utilised for ~ 20 ppm, pH 7 scale in 90 mins. This photocatalytic technology for RhB dye degradation seems to be a successful result. The Impact order of the rest interactions on RhB dye removal was depicted by the degradation rate described in the curves. In conclusion, this advanced and alternative approach to remove RhB dye with waste by-product or wood, i.e., sawdust, converted Biochar with improved catalytic materials, can be scaled up and has an advanced and alternative treatment option, and has the potential for other textile wastewater pollutants remediation. DLS results confirm that the degradation treatment alters both the size and distribution of suspended species, producing a more polydisperse mixture that includes smaller degraded fragments mixture that includes smaller degraded fragments and suddenly formed aggregates. Materials show Good magnetic behaviour (Ms) = ~ 24.59 emu/g and an easily separable response shown in VSM data. This study provides AOPs catalysts and plays an important role in promoting the development of AOPs. This original research can provide experimental experience and theoretical insights for the development of high-performance AOPs catalysts using cheap biomass resources, and contribute to the realisation of resource value-added while improving the efficiency and environmental friendliness of advanced oxidation processes.

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