Valorization of chicken bone waste into biochar adsorbent for methyl red adsorption from aqueous systems

Abstract

The sustainable valorization of poultry by-products into functional materials offers an eco-friendly approach for wastewater remediation and waste management. In this work, chicken bone waste (CBW) was converted into biochar (CBWB) via controlled pyrolysis for 2 h at 750 °C and examined as a low-cost adsorbent for the uptake of methyl red (MR) dye from aqueous solutions. The physicochemical characteristics of CBW and CBWB were evaluated using Fourier transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET) surface area analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Pyrolysis greatly improved porosity and surface area, increasing the BET surface area from 37.3 m²/g for CBW to 76.4 m²/g for CBWB. Batch sorption studies examined the impacts of solution pH, initial dye concentration, adsorbent dosage, contact time, and temperature on MR elimination. Adsorption kinetics followed the pseudo-first-order model for CBW and the pseudo-second-order model for CBWB, signifying stronger surface interactions after pyrolysis. Equilibrium data were best explained by the Langmuir isotherm for CBW and the Freundlich isotherm for CBWB, with maximum adsorption capacities of 84.84 and 138.58 mg/g, respectively. Thermodynamic assessment confirmed that the adsorption process was spontaneous and endothermic. Regeneration studies showed that CBWB reserved approximately 69% of its initial adsorption efficiency after five cycles of operation. These findings revealed that chicken bone waste–derived biochar is reusable, efficient, and sustainable adsorbent for dye-contaminated wastewater treatment.

Introduction

The swift growth of the global population and accelerating industrialization have led to a substantial rise in environmental pollution, with water contamination emerging as one of the most serious challenges. Among the major contributors to water pollution are synthetic dyes discharged from industries like cosmetics, textiles, pharmaceuticals, plastics, paper, and printing¹. During dyeing operations in the industry, it has been reported that approximately 15–20% of the dyes used are released into wastewater, causing severe contamination of aquatic environments². These dyes are chemically stable, visually persistent, and often toxic, making their elimination from water systems very difficult. Many synthetic dyes are carcinogenic and mutagenic, posing stern threats to human health and aquatic organisms^1–3. Their presence in water bodies lessens light penetration, overpowering photosynthetic activity and then reducing dissolved oxygen levels, which are crucial for sustaining aquatic ecosystems^3,4. Human exposure to dye-contaminated water can lead to allergic reactions, skin irritation, and respiratory challenges⁵.

Methyl red (MR) is a extensively used azo dye in fabric and textile dyeing applications. It contains an azo (–N = N–) chromophore linked to aromatic rings, a structural property that confers high resistance to biodegradation as a result of chemical stability^6,7. Exposure to MR through ingestion or inhalation may cause irritation of the eyes, skin, and gastrointestinal tract⁵. Furthermore, MR has been classified as a mitotic toxin, mutagen, and carcinogen, signifying its potential to bring about genetic mutations, disruption of cellular division, and increase cancer risk⁸. The toxicity and persistence of MR calls for the urgent need for sustainable, efficient, and cost-effective approaches for its elimination from wastewater prior to environmental release⁹.

Conventional dye elimination processes including chemical precipitation, coagulation–flocculation, ozonation, and membrane filtration, have been widely employed for wastewater dealing. Though, these approaches often suffer from weaknesses like high chemical consumption, membrane fouling, generation of secondary pollutants, and elevated costs of operations^10,11. In contrast, adsorption has developed as one of the most economical and effective methods for dye elimination because of its high efficiency, simplicity, flexibility in design, and minimal sludge generation^11–14. The effectiveness of adsorption procedures largely depends on the physicochemical characteristics of the adsorbent, mostly pore structure, surface area, and surface binding groups, which govern the capacity and affinity for contaminant uptake^10,11.

Traditional adsorbents like activated carbon and silica gel are effective but are restricted by high costs of production and limited reusability, encouraging rising interest in alternative low-cost materials^15,16. Biochar, most especially those derived from waste biomass, has added considerable interest because of its large surface area, porous structure, economic and environmental advantages, and surface functional groups availability¹⁷. Animal-waste-derived biochars are specifically attractive due to their abundance, inherent mineral content, and low cost which can enhance adsorption performance.

Recent research has explored a variety of animal and agricultural wastes such as eggshells¹⁰, fish bones¹⁸, and cattle bones¹⁹, as precursors for biochar preparation. These materials possess porous frameworks and are rich in calcium phosphate favorable for adsorption. Fundamental adsorption studies emphasize that porosity, surface chemistry, and interaction mechanisms play crucial roles in determining adsorption efficiency²⁰. Plant-based biochars have demonstrated promising performance for MR elimination, with adsorption strongly impacted by surface functional groups and pHpzc values²¹. Likewise, bone-derived hydrochars have shown effective MR adsorption, often following the Freundlich isotherm and governed by electrostatic interactions, as corroborated by zeta potential analyses⁷. These reports underscore the growing potential of low-cost, sustainable adsorbents and highlight the essential of material design and mechanistic understanding in improving adsorption performance²².

For example, Atmani et al.²³ and Akkari et al.²⁴ reported that waste-derived and bio-based adsorbents are gaining increasing attention because of their environmental compatibility and cost-effectiveness. Mineral-based composites and surface-engineered carbonaceous have further improved adsorption performance via enhanced surface chemistry and porosity²⁵. Photocatalytic and optically active materials have also been utilized for dye degradation²⁶; however, their large-scale application is often inhibited by catalyst recovery challenges, high synthesis costs, and dependence on external energy sources.

Environmental sustainability has further driven the conversion of agricultural wastes and industrial into functional adsorbents. Research conducted by Akkari et al.²⁷ and Kaci et al.²⁸ highlighted the importance of waste valorization for decontaminating emerging pollutants and dyes from water, concurrently addressing waste disposal problems and reducing treatment costs. Previous studies by Khatoon et al.²⁹ noted that cellulose extracted from Luffa acutangula peels showed effective adsorption performance for crystal violet, methylene blue, and diclofenac sodium, emphasizing the value of agricultural residues as adsorbents. In addition, Isaac et al.³⁰ reported that magnetic biochars derived from agro-waste of Juglans regia shells exhibited great efficient removal capacity towards heavy metal ions, reinforcing the potential for engineered biochars to enhance adsorption capacity via magnetic functionalization and structural modification. Notwithstanding these advances, the study of underutilized animal-derived wastes, most especially poultry bone waste, remains limited.

Chicken bone waste is a major by-product of the poultry industry and is generated in huge quantities worldwide, creating substantial environmental and waste management challenges³¹. Improper disposal methods, such as landfilling and open dumping, often result in odor emissions, microbial contamination, and environmental degradation³². While comprehensive national data on bone waste alone are limited, Nigeria’s poultry and slaughter sectors generate a substantial volume of animal and poultry waste that includes bone fractions. Many poultry farms report waste generation ranging from approximately 0.09 kg to 0.18 kg of waste per bird depending on farm size, which, when multiplied across Nigeria’s extensive poultry population, translates to tens of thousands of tons of poultry waste annually³³. Moreover, broader abattoir data indicate that bones constitute a great proportion of livestock processing by-products, with bones representing up to 40% of total slaughter by-products in Nigerian slaughterhouses, signifying that bone waste streams are substantial and under-utilised³⁴. In terms of current management practices, poultry and slaughterhouse wastes in Nigeria are often improperly managed or under-valorised as surveys indicate frequent disposal methods like burning, burying, or infrequent collection, with only a fraction of waste being repurposed for manure or other uses³⁵. Converting chicken bone waste into value-added materials not only mitigates these environmental concerns but also supports circular economy initiatives by transforming waste into a functional product.

Chicken bone waste is primarily composed of hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) embedded within a carbonaceous matrix, providing unique advantages for adsorbent development³⁶. Pyrolysis enhances the porosity, surface area, and stability of chicken bone waste, while the hydroxyapatite component facilitates ion exchange and electrostatic interactions. The carbonaceous phase contributes oxygen-containing functional groups such as hydroxyl, carboxyl, and phosphate groups, enabling strong interactions with dye molecules via hydrogen bonding, electrostatic attraction, and π–π interactions^32,37.

Despite its promising features, studies focusing on chicken bone waste–derived biochar for methyl red removal remain scarce. Most existing works emphasize plant-based biochars or chemically modified carbons, leaving a research gap in the valorization of poultry waste for dye adsorption. Moreover, systematic investigations comparing raw bone waste and its biochar derivative under identical conditions are limited. Addressing this gap is very relevant in developing regions like Nigeria, where poultry and slaughterhouse wastes are generated in significant amount but are often poorly managed^33–35. While different bone-derived adsorbents, like hydroxyapatite, bone ash, and hydrochar, have been examined for dye removal, most works focus on either non-carbonized bone materials or mineral-rich phases. Whereas, chicken bone waste biochar prepared through controlled pyrolysis combines carbonized aromatic structures with hydroxyapatite mineral phases, producing a multifunctional adsorbent with both carbonaceous and inorganic active sites. This unique hybrid features permits diverse adsorption mechanisms, including ion exchange, hydrogen bonding, electrostatic attraction, and π–π interactions, differentiating CBWB from conventional bone-derived adsorbents. Thus, this study aims to valorize chicken bone waste into biochar via controlled pyrolysis and to systematically evaluate the adsorption performance of raw chicken bone waste (CBW) and chicken bone waste biochar (CBWB) for methyl red elimination from aqueous solutions. The effects of key parameters including initial dye concentration, contact time, pH, adsorbent dosage, and temperature were studied. Adsorption kinetics were analyzed using pseudo-first-order, pseudo-second-order, and intraparticle diffusion models, while equilibrium behavior was studied using Langmuir, Freundlich, and Dubinin–Radushkevich isotherm models. Thermodynamic parameters were computed to elucidate the nature of the adsorption process, and regeneration studies were conducted to assess the reusability and practical applicability of the adsorbents for sustainable wastewater treatment.

Materials and methods

Materials triplicate

Chicken bone waste (CBW) was collected from local poultry outlets in Ota, Ogun State, Nigeria. The waste was washed thoroughly with distilled hot water to remove adhering flesh, fat, and impurities, followed by oven-drying at 100 °C for 6 h to eliminate moisture. The dried bones were then crushed using a mortar and pestle and sieved to obtain a uniform particle size. Methyl red (C₁₅H₁₅N₃O₂, molecular weight = 269.3 g/mol, ≥ 99% purity) obtained from BDH, London, was used as the model dye pollutant. Hydrochloric acid (HCl, 37%, analytical grade) and sodium hydroxide (NaOH, ≥ 98%, analytical grade) were used for pH adjustment. All chemicals and reagents used were of analytical grade and utilized without further purification. Distilled water was used throughout the experiments.

Methyl red solution and chicken bone waste biochar (CBWB) preparations

A stock solution of methyl red dye (1000 mg/L) was prepared by dissolving 1 g of the dye in 1 L of distilled water. Working solutions with concentrations of 50, 100, 150, 200, 250, and 300 mg/L were obtained through appropriate serial dilutions of the stock solution. The pH of each solution was adjusted using 0.1 M HCl or 0.1 M NaOH as required. Chicken bone waste was transformed into biochar through controlled thermal treatment. About 70 g of the dried and pulverized CBW was placed in a covered ceramic crucible and heated in a muffle furnace for 2 h at 750 °C using a heating rate of 10 °C/min. The crucible cover was used to limit oxygen inflow, thereby creating a limited-oxygen environment suitable for carbonization and averting complete combustion of the bone biomass. After thermal activation, the furnace was allowed to cool to room temperature, after which the resulting biochar was pulverized and sieved to obtain a fine powder. The developed material was stored in airtight containers and labeled as chicken bone waste biochar (CBWB). Figure 1 illustrates the conversion of chicken bone waste into biochar adsorbent.

Fig. 1.

Schematic representation of conversion of chicken bone waste into biochar adsorbent.

Determination of point of zero charge (pH_pzc)

The point of zero charge (pH_pzc) of CBW and CBWB was evaluated using the pH drift method as previously described by Appiah-Brempong et al.³⁸ and Njewa et al.³⁹. Briefly, 50 mL of 0.01 M NaCl solution was placed in a series of closed conical flasks, and the initial pH (pH_i) was regulated between 2 and 12 using 0.1 M HCl or 0.1 M NaOH. Thereafter, 40 mg of adsorbent was added to the solutions, which were then agitated for 24 h at 150 rpm and room temperature to reach equilibrium. After equilibration, the final pH (pH_f) was computed. The pH_pzc was estimated from the plot of ΔpH (pH_f − pH_i) versus pH_i, where ΔpH = 0.

Characterization of adsorbents

The raw chicken bone waste (CBW) and chicken bone waste biochar (CBWB) were characterized to determine their structural properties. Fourier Transform Infrared (FT-IR: Bruker Tensor 27 spectrophotometer, US) spectroscopy was used to identify the surface functional groups responsible for adsorption within the range of 4000–400 cm⁻¹. The surface morphology of the adsorbents was examined using Scanning Electron Microscopy (SEM: Carl Zeiss, Jena, Germany), while the specific surface area, and pore diameter were analyzed using Brunauer–Emmett–Teller (BET) surface area analysis at 77 K. The X-ray Diffraction (XRD: PANalytical, X’Pert PRO, Netherland) analysis was conducted using Cu–Kα radiation (λ = 1.5406 Å) operated at 40 kV and 30 mA, with a 2θ scanning range of 10°–65° to identify crystalline phases and structural transformations. Thermogravimetric Analysis (TGA: TGA 4000, Perkin Elmer, Haarlem, Netherlands) was performed under a nitrogen atmosphere with a heating rate of 10 °C/min from room temperature to 800 °C to study the thermal stability and decomposition behavior of the samples. The surface area and pore size distribution were performed using Brunauer–Emmett–Teller (BET) analysis with a Quantachrome NOVA 2200C (USA).

Adsorption experiments

Sorption experiments were performed to examine the influence of key operational parameters like contact time, adsorbent dosage, solution pH, initial dye concentration, and temperature on the removal of methyl red (MR) by CBW and CBWB. For each experiment, a known mass of adsorbent (10–100 mg) was added to MR solutions of known concentrations in 250 mL conical flasks. The initial pH of the solutions was regulated within the range of 2–12 using 0.1 M HCl or 0.1 M NaOH before the addition of the adsorbent and monitored using a calibrated pH meter. The suspensions were thereafter agitated on an orbital shaker at 150 rpm within selected temperatures (25–60 °C) for varying contact times (10–200 min) to achieve adsorption equilibrium. Upon the completion of each experiment, the mixtures were filtered, and the residual MR concentration in the filtrate was measured with a UV–Vis spectrophotometer at 430 nm. All experiments were conducted in triplicate under identical conditions to ensure reproducibility, and average values were employed for analysis of the data. The adsorption capacity at equilibrium (q_e, mg/g) and the percentage removal (%R) were calculated using Eqs. (1) and (2), respectively.

1

2

where C_o and C_e are the initial and equilibrium dye concentrations (mg/L), w is the solution volume (L), and m is the mass of adsorbent (g). The batch studies were conducted thrice and the average of the replicates were recorded. Experimental uncertainties were measured using standard deviation, which was obtained to be within ± 5% for most measurements, indicating good consistency of the data.

Desorption experiments

Desorption studies were performed to assess the reusability of the two biomasses. Acetic acid was selected as the desorbing agent because of its mild acidity and effectiveness in reversing dye adsorption without damaging the adsorbent structure, as similarly reported in previous adsorption–desorption studies⁴⁰. Preliminary desorption experiments were first performed using different desorbing solvents such as HCl, NaOH, acetic acid, and distilled water. The adsorbent previously loaded with MR were treated with 0.2 M acetic acid as the desorbing agent. A 0.2 M acetic acid solution was chosen as the desorbing agent due to its mild acidity, effectiveness, and compatibility with the adsorbent structure. Acetic acid was favored since it provides the best desorption percentage and demonstrated enough proton concentration to reverse dye adsorption without causing extreme dissolution of the hydroxyapatite phase or structural degradation of the biochar matrix. The concentration chosen (0.2 M) represents an optimized balance between adsorbent stability and desorption efficiency, permitting effective dye recovery while preserving the structural integrity and reusability of the adsorbent over multiple cycles. The recovered MR were quantified, and the regenerated biochar was subsequently reused for further adsorption–desorption cycles. Specifically, 40 mg of spent adsorbent was equilibrated with 25 mL of the pollutant solution for 100 min at 150 rpm. After equilibration, the suspension was filtered and the residual MR concentration was determined using UV–Vis spectrophotometer. This procedure was repeated for five successive cycles under identical conditions. The percentage desorption was calculated using the following expression:

3

where D represents desorption efficiency, AD is the amount desorbed, and AA is the amount initially adsorbed.

Adsorption kinetics

The adsorption kinetics were modeled using the pseudo-first-order, pseudo-second-order, intraparticle diffusion, and Elovich models to determine the rate-controlling step and adsorption mechanism.

Pseudo-first-order model^7,21,40:

4

where k₁ (min⁻¹) is the pseudo-first-order rate constant.

Pseudo-second-order model^41,42:

5

where k₂ (g mg⁻¹ min⁻¹) is the pseudo-second-order rate constant.

Intraparticle diffusion model^43,44:

6

where k_id (mg g⁻¹ min⁻¹˙⁵) is the intraparticle diffusion rate constant, and C (mg g⁻¹) represents the boundary layer thickness.

To further study the adsorption mechanism, the kinetic data were also subjected to Elovich model, which is commonly used for systems involving chemisorption on heterogeneous surfaces. The Elovich equation is expressed as⁴⁵:

7

where (mg g⁻¹) is the amount of methyl red adsorbed at time , (mg g⁻¹ min⁻¹) represents the initial adsorption rate, and (g mg⁻¹) is related to the extent of surface coverage and activation energy for chemisorption.

The best-fitting model was determined based on the correlation coefficient (R²) and the sum of squares error (%SSE)¹¹:

8

Adsorption isotherm studies

The adsorption isotherm models were employed to describe the equilibrium relationship between the amount of methyl red adsorbed onto the adsorbents and its equilibrium concentration in solution. The equilibrium data were analyzed using the Langmuir, Freundlich, and Dubinin–Radushkevich (D–R) isotherm models to elucidate the adsorption mechanism and surface characteristics of the adsorbents.

Langmuir isotherm

The Langmuir model assumes monolayer adsorption on a homogeneous surface with finite, identical sites and is expressed as^31,44:

9

where C_e (mg/L) is the equilibrium concentration of dye, q_e (mg/g) is the equilibrium adsorption capacity, q_max (mg/g) represents the maximum monolayer adsorption capacity, and K_L (L/mg) is the Langmuir constant related to adsorption energy.

The dimensionless separation factor (R_L) was used to determine the favorability of adsorption^10,31:

10

Freundlich isotherm

The Freundlich isotherm describes adsorption on heterogeneous surfaces with non-uniform energy distribution and is given by^10,46:

11

where K_f​ ((mg/g)(L/mg)^1/n) is the Freundlich constant indicating adsorption capacity, and 1/n is the heterogeneity factor that provides information on adsorption intensity. The best-fitting model, as determined by the correlation coefficient (R²) and other error functions, was used to describe the adsorption mechanism and surface affinity of methyl red onto CBW and CBWB.

Dubinin–Radushkevich (D–R)

The D–R model can be deploy to evaluate the nature of the adsorption process be it physical or chemical and estimates the adsorption energy^11,31:

12

The D–R isotherm can be use to elucidates the mechanism of adsorption by computing the mean free energy E and Polanyi potential constant, ϵ presented^11,31:

13

14

Thermodynamic studies

The thermodynamic behavior of methyl red adsorption onto the prepared adsorbents was evaluated to better understand the energy changes and feasibility of the process. Key thermodynamic parameters, including the Gibbs free energy (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°), were determined. These parameters provide valuable insights into whether the adsorption occurs spontaneously, the nature of the interactions involved, and the degree of structural disorder at the solid–liquid interface.

is the dimensionless equilibrium constant, obtained by normalization of the experimental distribution coefficient. The corresponding thermodynamic equations are presented below^47–49:

15

16

17

Results and discussion

Effect of contact time and initial concentration on methyl red adsorption

The sorption of methyl red onto chicken bone waste and chicken bone waste biochar was examined at various contact times (0–200 min) and initial dye concentrations (50–300 mg/L) as depicted in Figs. 2 and 3 respectively. In both cases, sorption was rapid during the initial 50–70 min, followed by a slower phase until equilibrium was attained at 120 and 150 min for CBW and CBWB respectively. This two-stage pattern suggests the availability of abundant active sites in the early stage and their gradual saturation over time. For instance, at 100 mg/L, CBW uptake rose from 10.3 mg/g at 10 min to 39.5 mg/g at equilibrium, while CBWB increased more sharply from 14.2 to 45.1 mg/g within the same period. Likewise, at 200 mg/L, equilibrium sorption was 68.6 mg/g for CBW and 88.4 mg/g for CBWB. The higher values noticed for CBWB confirm that pyrolysis enhances surface area, pore structure, and accessibility of active sites. Similarly, the initial dye concentration strongly enhanced the sorption performance for both materials. With surge in concentration, adsorption capacity consistently rose, driven by a stronger concentration gradient that influenced mass transfer from solution to adsorbent surfaces. At equilibrium, CBW exhibited maximum uptakes of 80.6 mg/g at 300 mg/L, while CBWB achieved substantially higher values of 132.4 mg/g at the same concentrations. The comparison reveals that while both adsorbents follow similar sorption kinetics—rapid initial uptake and gradual equilibration, CBWB consistently outperforms CBW. The higher adsorption capacity reported for CBWB compared to CBW affirms that pyrolysis significantly enhances the physicochemical properties of chicken bone waste. This enhancement is primarily due to the increased surface area and the development of a porous carbonaceous matrix as demonstrated by BET and SEM analyses. Thermal treatment eliminates organic matrix such as collagen and promotes pore formation, thus increasing the accessibility of the sorption sites and improving dye–adsorbent interactions. Similar improvements in adsorption performance after pyrolysis of bone-derived materials have been widely reported in the literature^50,51.

Fig. 2.

Influence of contact time and initial methyl red concentration (50–300 mg/L) on the adsorption of methyl red onto raw chicken bone waste (CBW) at: adsorbent dosage = 60 mg, solution pH = 8.0, temperature = 298 K, agitation speed = 150 rpm, and contact time range = 0–200 min.

Fig. 3.

Influence of contact time and initial methyl red concentration (50–300 mg/L) on the adsorption of methyl red onto chicken bone waste biochar (CBWB) at: adsorbent dosage = 50 mg, solution pH = 8.0, temperature = 298 K, agitation speed = 150 rpm, and contact time range = 0–200 min.

These findings are consistent with earlier works reporting that biochar derived from bone waste demonstrates superior sorption performance when compared to their raw counterparts because of improved physicochemical characteristics^21,52. For instance, using bali cow bones hydrochar powder, Neolaka et al.⁵² showed that maximum MR sorption was achieved to be 7.2 mg/g at 30 min. Wolski et al.⁵¹ noted that bio-carbons from biogas plant wastes demonstrated that methyl red sorption capacities in the range of 30–113 mg/g was attained which was said to have depended on contact time, initial concentration, and adsorbent dose. The faster attainment of equilibrium noticed for CBW compared to CBWB can be attributed to variations in surface structure, surface chemistry and pore diffusion^21,51–53. While CBWB shows a higher overall adsorption capacity, CBW achieved equilibrium more rapidly and this can be as a result of the differences in surface accessibility and mass transfer mechanisms. The raw CBW contains readily exposed receptor groups like phosphate and calcium-based fractions, which enable dye molecules to interact directly with surface sites, ensuing in faster adsorption kinetics. On the other hand, the CBWB possesses a heterogeneous pore structure with abundant micro- and mesopores. While these pores significantly increase adsorption capacity, dye molecules must diffuse into the internal pore network, introducing intraparticle diffusion resistance and therefore prolonging the time required to reach equilibrium. This trend aligns with the observed kinetic behavior and consistence with reports on porous biochar-based adsorbents^7,51.

Although this influences total sorption capacity, the dye molecules must diffuse deeper into the pores, causing slower mass transfer and delayed equilibrium. In addition, pyrolysis alters surface chemistry by eliminating some easily accessible polar groups (–OH, –COOH) as seen from the FT-IR graph while enriching aromatic carbon domains. These carbonized surfaces enhance hydrophobic interactions and π–π stacking that increase dye uptake, but the kinetics are relatively slower when compared with the rapid ionic binding seen in the case of raw CBW. Wolski et al.⁵¹ observed that biogas-plant-derived biochars had delayed equilibrium when compared with raw feedstock, which was attributed to diffusion into micropores. Liu et al.⁵³ found out that the sorption of lincomycin onto biochars was followed by a relatively slow intraparticle diffusion into internal biochar pore structures.

Effect of adsorbent dosage

When we varied the dosage of CBW and CBWB, it was noticed that more adsorbent generally improves the uptake of MR, however, only up to a point as revealed in Fig. 4. The results show that with a surge in the adsorbent dosage, the percentage removal of methyl red increased for both materials because of the greater availability of active binding sites and increased surface area⁵⁴. For CBW, the removal efficiency surged from 44.9% at 10 mg to 70.9% at 60 mg, after which no meaningful improvement was observed. Likewise, CBWB displayed a more pronounced increase, rising from 53.9% at 10 mg to 85.9% at 50 mg, followed by a plateau at higher dosages. The observed leveling-off at higher dosages can be ascribed to saturation of available dye molecules relative to the number of sorption sites, and possible particle agglomeration, which may diminish the effective surface area^54,55. Such feature has been extensively reported in adsorption studies and suggests that an optimal adsorbent dosage exists beyond which further increases do not improve removal efficiency. These results reveal that CBWB requires a lower dosage than CBW to attain higher removal efficiency, indicating its enhanced porosity and surface properties following pyrolysis. For example, Priyanka et al.⁵⁴ reported improved dye elimination during wastewater remediation using mesoporous activated wheat straw biochar, where the removal efficiency increased with increasing biochar dosage. But, beyond an optimal dosage, additional increases did not lead to significant improvement in adsorption, which was ascribed to the saturation of available surface-active sites. Likewise, Le et al.⁵⁵ noticed that methyl red removal increased with increasing biochar dosage up to a certain threshold, after which additional biochar produced lessening adsorption gains. In general, increasing the adsorbent dosage offers a greater number of available binding sites, allowing more dye molecules to be removed from solution until site saturation is reached.

Fig. 4.

Influence of adsorbent dosage (10–100 mg) on the sorption of methyl red onto CBW and CBWB at: initial dye concentration = 300 mg/L, solution pH = 8.0, temperature = 298 K, contact time = 120 min (CBW) and 150 min (CBWB), and agitation speed = 150 rpm.

Role of effect of pH and point of zero charge (pH_pzc)

Surface charge characterization using pH_pzc and zeta potential is widely recognized as a vital parameter for corroborating electrostatic contributions to contaminant adsorption, as indicated in recent studies correlating surface charge evolution with adsorption efficiency⁵⁶. The experimental results for both CBW and CBWB clearly demonstrate that methyl red uptake increases progressively as the solution pH shifts from strongly acidic (pH 2) toward neutral and mildly alkaline conditions (pH 7–8), as shown in Fig. 5. Specifically, CBW adsorption surge from 46.5% at pH 2 to 73.3% at pH 8, while CBWB exhibits a corresponding increase from 53.3 to 88.1% over the same pH range. Outside pH 8–10, adsorption levels off, displaying only marginal further improvement. This pH-dependent adsorption characteristics is strongly related to the point of zero charge (pH_pzc) of the adsorbents, defined as the pH at which the net surface charge is zero^38,39. At pH values lower than the pH_pzc, the adsorbent surface is mostly positively charged, whereas at pH values above the pH_pzc, the surface gains a net negative charge^38,39. For bone-derived materials like raw bone and bone char in this present study, the obtained pH_pzc values are 5.5 and 6.2. respectively. Consequently, at solution pH values lower than the pH_pzc, electrostatic interactions between the surface and ionizable dye species are less favorable, whereas increasing the pH above the pH_pzc improves electrostatic attraction and promotes greater dye uptake. This explains the pronounced increase in methyl red uptake noticed at near-neutral to alkaline pH.

Fig. 5.

Influence of solution pH (2–12) on the sorption of methyl red onto CBW and CBWB at: initial dye concentration = 300 mg/L, adsorbent dosage = 60 mg, temperature = 298 K, contact time = 120 min (CBW) and 150 min (CBWB), and agitation speed = 150 rpm.

These findings are in accordance with earlier work by Appiah-Brempong et al.³⁸, who exxamined bone char derived from cattle head bones for color and COD elimination from vegetable tannery wastewater and demonstrated that solution pH strongly influences sorption behavior due to pH-dependent surface charge variations relative to pH_pzc. Moreover, the study reported by Manzar et al.⁵⁶ explicitly shows how pH_pzc values govern adsorbent surface charge states and, subsequently, adsorption of ionizable contaminants. Similarly, Ahmad et al.⁵⁷ provides zeta potential measurements that is consistent with the critical function of surface charge evolution with pH in governing electrostatic interactions between charged dye molecules and adsorbents.

Effect of temperature on MR adsorption

As depicted in Fig. 6, raising the temperature led to enhanced sorption of methyl red by both CBW and CBWB adsorbents. It was observed that as temperature rose, the sorption efficiency improved, though CBWB consistently outperformed CBW across all thermal conditions with maximum uptake of 69.9% at 45 °C and 81.2% at 50 °C. This enhancement could be as a result of higher kinetic energy of dye molecules at elevated temperatures, which enhances diffusion through the liquid boundary layer and penetration into internal pores of the adsorbents⁵⁸. For CBW, the increase was modest, indicating its limited pore accessibility, whereas in the case of CBWB, it displayed a more pronounced rise in adsorption, benefiting from the induced structural modifications by pyrolysis. This observation indicates that the sorption process is endothermic, which requires thermal energy to improve dye–adsorbent interactions. Similar patterns were reported by Paluch et al.⁵⁹ where it was observed that methyl red adsorption onto fennel-seed-derived biochar surged substantially with temperature. Similarly, Teweldebrihan and Dinka²¹ reported higher sorption capacities of rumex abyssinicus biochar for MR uptake at higher temperatures, consistent with an endothermic mechanism. A study carried on snow-lotus residue biochar by Zhang et al.⁵⁸ also indicated that elevated temperature improved sorption performance by enhancing dye diffusion and activating surface functional groups.

Fig. 6.

Influence of temperature (298–323 K) on the sorption of methyl red onto CBW and CBWB at: initial dye concentration = 300 mg/L, adsorbent dosage = 60 mg, solution pH = 8.0, contact time = 120 min (CBW) and 150 min (CBWB), and agitation speed = 150 rpm.

Kinetics and mechanistic implications

To evaluate the adsorption dynamics of MR, the kinetic data for CBW and CBWB were fitted to pseudo-first-order, pseudo-second-order, Elovich and intraparticle diffusion models (Fig. 7a, b, c and Table 1). A direct relationship between the experimental adsorption capacities (q_e,exp) and the calculated values (q_e,cal) further authenticates the suitability of the applied kinetic models (Table 1). In addition to the correlation coefficient (R²), the sum of squared error (%SSE) was used to evaluate the goodness-of-fit of the kinetic models (Table 1). It is important to note that low SSE values observed, alongside noticeable higher R² values suggests suitable model fitting. In the case of the CBW, the pseudo-first-order model demonstrates a close agreement between q_e,exp and q_e,cal across all initial dye concentrations. For example, at an initial concentration of 100 mg/L, q_e,exp was 39.50 mg/g which closely matches the calculated value (40.70 mg/g), while small deviations are noticed at higher concentrations. A comparative evaluation of %SSE and R² values (Table 1) reveals that for CBW, the pseudo-first-order model exhibited higher R² values (0.978–0.998) alongside very low %SSE (0.001–0.031), indicating excellent agreement with experimental data, signifying a physisorption-dominated process^7,21. Although the pseudo-second-order model also showed low %SSE (0.017–0.097), its slightly lower R² suggests a comparatively weaker fit.

Fig. 7.

Kinetics parameters of (a) pseudo-first-order, (b) pseudo-second-order, (c) intraparticle diffusion, and (d) Elovich models on the adsorption of methyl red dye via chicken bone waste and biochar adsorbents.

Table 1.

Kinetics parameters values of the adsorption of methyl red dye via chicken bone waste and biochar adsorbents.

		First order				Second order				Intra particle diffusion			Elovich
Co (mg/L)	Qe(exp) (mg g−1)	Qe cal (mg g−1)	k1 (min−1)	R2	% SSE	Qe cal (mg g−1)	k2 (g mg−1 min−1)	R2	% SSE	Kp (mg g-1 min−0.5)	C1 (mg g−1)	R2	α	β	R2
CBW
50	19.300	21.287	0.012	0.987	0.031	25.489	0.055	0.935	0.097	2.687	0.599	0.985	0.025	0.103	0.836
100	39.500	40.699	0.019	0.996	0.009	41.727	0.069	0.936	0.017	4.060	0.791	0.977	0.031	0.116	0.915
150	55.000	57.512	0.037	0.978	0.014	63.041	0.073	0.928	0.044	7.806	1.277	0.995	0.052	0.135	0.925
200	68.600	69.103	0.053	0.995	0.002	75.732	0.085	0.896	0.031	8.658	1.369	0.975	0.083	0.202	0.877
250	73.400	71.002	0.072	0.998	0.010	84.107	0.116	0.947	0.044	10.858	3.432	0.986	1.016	0.214	0.847
300	80.600	82.380	0.075	0.996	0.007	89.700	0.168	0.894	0.034	13.222	5.268	0.996	1.124	0.301	0.922
CBWB
50	23.700	29.299	0.022	0.897	0.071	24.025	0.230	0.996	0.004	1.718	0.1	0.995	0.552	0.125	0.956
100	45.100	34.864	0.023	0.897	0.068	43.484	0.288	0.996	0.011	3.230	0.480	0.994	0.684	0.154	0.979
150	68.900	57.501	0.039	0.918	0.050	69.089	0.355	0.987	0.001	5.087	0.753	0.988	1.018	0.207	0.987
200	88.400	69.747	0.053	0.938	0.064	91.135	0.428	0.997	0.009	6.022	1.055	0.996	2.229	0.024	0.987
250	116.600	98.900	0.079	0.889	0.046	113.227	0.590	0.978	0.009	7.666	1.220	0.995	3.031	0.301	0.972
300	132.400	111.305	0.082	0.919	0.048	133.011	0.636	0.995	0.001	9.794	1.899	0.977	4.018	0.360	0.977

In contrast, in the case of the CBWB, the pseudo-second-order model offers a significantly better alignment between q_e,exp and q_e,cal. For instance, at 200 mg/L, the q_e,exp measured capacity was 88.40 mg/g which is very close to the q_e,cal value (91.14 mg/g), and at 300 mg/L, q_e,exp (132.40 mg/g) closely matches q_e,cal (133.01 mg/g). The pseudo-second-order model significantly improved both parameters, yielding lower %SSE (0.001–0.011) and higher R² values (0.987–0.997), confirming its suitability. These observations indicate that while %SSE values are generally low due to normalization, the relative differences in %SSE combined with R² provide a more reliable basis for model selection. Consequently, the pseudo-first-order model best describes CBW adsorption kinetics, whereas the pseudo-second-order model is more appropriate for CBWB.

To further illuminate the adsorption mechanism and rate-controlling steps, kinetic examination was extended beyond the frequently used pseudo-first-order and pseudo-second-order models. In addition to these classical methods, the Elovich and intraparticle diffusion models were engaged to provide a more inclusive interpretation of the sorption process and the likely contribution of multiple transport and reaction phenomena. The intraparticle diffusion plots for both adsorbents show high linearity with R² in the range of 0.97–0.99 but nonzero intercepts, implying that internal diffusion contributes significantly but is not the sole rate-limiting step. Comparing the two adsorbents, CBW appears to obey a physisorption-dominated mechanism (consistent with first-order kinetics), while CBWB exhibits stronger adsorption behavior or mixed involving specific chemical interactions, which is consistent with better fit to the pseudo-second-order model. This dual contribution is further corroborated by the high intraparticle diffusion coefficients (K_p) and the concentration dependence of K_p in CBWB, indicating that as dye load surge, diffusion into pores becomes more pronounced. Comparatively, observations from literature have been made in adsorption studies of dyes onto modified biochar, where the pseudo-second-order model often fits best but intraparticle diffusion also plays a vital role as well^60,61.

The Elovich model demonstrated a strong fit for adsorption on MR by both CBW and CBWB, as indicated by the high R² values. In the case of CBW, the R² values ranged from 0.836 to 0.925, while those for CBWB were much higher and fall within the range of 0.956–0.987. This signifies that adsorption of MR by CBWB follows Elovich kinetics much better than CBW, suggesting a chemisorption-dominated mechanism occurring on energetically heterogeneous surfaces, with CBWB showing a marginally superior conformity to the model. The lower values R2 for CBW imply that a physisorption mechanism is more prefers. The initial adsorption rate constant (α) was significantly higher for CBWB (0.552–4.018) compare with CBW (0.025–1.124), indicating a much faster initial uptake of the MR molecules by the biochar. This enhancement could be as a result of the biochar formation, which improves porosity, surface area, and the availability of reactive adsorption sites^47–49, while the rising of α values further suggest improved adsorption efficiency under the studied conditions^60,61. The parameter that is associated with surface heterogeneity and the activation energy of chemisorption (β), was found to range from 0.103 to 0.301 for CBW and from 0.024 to 0.36 for CBWB. The lower minimum and wider spread of β values seen in the case of CBWB signify a more heterogeneous surface with a broader distribution of adsorption energies, promoting stronger adsorbate–adsorbent interactions at specific sites^45,62.

The evaluations of similar multi-model kinetic have been successfully applied to described dye adsorption onto modified biosorbents, revealing multi-step adsorption behavior that cannot be adequately captured by a single kinetic model⁶². Studies on sustainable adsorbents derived from agricultural and industrial wastes by Atmani et al.⁶³ have likewise emphasized the significance of using multiple kinetic models to precisely interpret adsorption dynamics, reinforcing that complex sorbate–sorbent interactions often swerve from simple first- or second-order kinetics. A recent study by Tiliouine et al.⁶⁴ into advanced surface-modified adsorbents have additionally highlighted the value of alternative kinetic expressions, including mixed kinetic approaches and heterogeneous reaction models, to explain multi-mechanistic uptake behavior. The wider relevance of employing complementary kinetic models and recent advances in adsorption kinetic modeling have also been documented for dye adsorption systems⁶⁵, whereas comparative contextualization of adsorption performance across similar sorption systems has been discussed in related studies⁶⁶.

Adsorption isotherms

The equilibrium data were examined using the Langmuir, Freundlich, and Dubinin–Radushkevich (D–R) isotherm models as presented in Fig. 8 and Table 2. For CBW, the Langmuir model exhibited the best fit with R² = 0.996, indicating monolayer adsorption on a homogeneous surface with a maximum capacity (Q_max) of 84.84 mg/g^10,11. The dimensionless separation factor (R_L = 0.52) falls between 0 and 1, suggesting favorable adsorption. The Freundlich constants (K_F = 40.24 mg/g, 1/n = 0.450) also point to favorable adsorption, though the slightly lower R² (0.946) indicates that multilayer or heterogeneous binding is less dominant. The D–R model gave a mean free energy values of E = 0.18 kJ/mol, which is below 8 kJ/mol, confirming a physisorption process^10,11,15. On the contrary, CBWB demonstrated superior adsorption capacity and different isotherm characteristics. In this case, the Langmuir model showed a significantly higher Q_max of 138.58 mg/g, confirming greater monolayer uptake capacity than CBW. The very low R_L value (0.014) suggests highly favorable adsorption at the studied concentrations. However, the low value of R² (0.868) indicates that this model is not ideal for describing the isothermal behaviour of MR uptake by CBWB. Interestingly, the Freundlich model fit CBWB exceptionally well with R² value of 0.998, with high K_F (96.67 mg/g) and low 1/n (0.230), indicating strong adsorption affinity and surface heterogeneity. The D–R model gave a mean free energy of only 9.020 kJ/mol, again consistent with chemisorption^10,11,15. CBWB aligns more strongly with the Freundlich model, inferring heterogeneous surface binding and higher adsorption affinity. This implies that thermal activation of the chicken bone waste created more diverse adsorption sites, that enhanced both its affinity and capacity compared with CBW.

Fig. 8.

Isotherms parameters of (a) Langmuir, (b) Freundlich, and (c) Dubinin–Radushkevich on the adsorption of methyl red dye via chicken bone waste and biochar adsorbents.

Table 2.

Isotherms parameters values of the adsorption of methyl red dye via chicken bone waste and biochar adsorbents.

Models	Parameters	CBW	CBWB
Langmuir	Qmax(mg/g)	84.84	138.58
	RL	0.52	0.014
	R2	0.996	0.868
	SSE	0.013	0.025
Freundlich	KF ((mol g−1) (mol L−1)−1/n	40.24	96.67
	1/n	0.450	0.230
	R2	0.746	0.998
	SSE	0.032	0.013
Dubinin–Radushkevich (D–R)	qm (mg/g)	67.2	98.17
	E (kJ/mol)	0.18	9.020
	R2	0.974	0.988
	SSE	0.041	0.036

Table 3 indicates a comparative comparison of the maximum adsorption capacities of other adsorbents reported in the literature for the uptake of methyl red dye with chicken bone wastes and chicken bone waste biochar. The current study shows that chicken bone waste (CBW), and its biochar derivative (CBWB) exhibits an exceptional adsorption capacity of 84.84 mg/g and 138.58 mg/g, surpassing all the earlier reported adsorbents. This improvement is linked to the natural mineral content and porosity of bone material. This highlights that thermal activation not only improves the surface area and pore volume but also increases the number of accessible active sites for dye interaction. The results emphasize that while a variety of agricultural and biological residues have been explored for methyl red adsorption, CBW and CBWB stand out as a highly efficient adsorbent.

Table 3.

Comparison of maximum adsorption capacities of different adsorbents for methyl red removal.

Adsorbent	Adsorption capacity (mg/g)	pH	Temperature	Initial Dye Concentration	Adsorbent Dose	Reference
Funnel seeds	26	N/A	N/A	N/A	N/A	Paluch et al.47
Biochar from Rumex Abyssinicus (RAB)	42.34	6	N/A	N/A	N/A	Teweldebrihan & Dinka,21
Hydrochar from Bali Cow Bones	7.2	6	30 °C (303 K)	20 mg /L	0.5 g	Neolaka et al.52
Carbon Bioadsorbent from Biogas Plant Waste (PA7)	31–113	~ 7.3	~ 22 °C	30 mg /L	30 mg	Wolski et al.51
Activated Carbons from Moringa oleifera Shells	28.67	~ acidic–neutral	25 °C	25 mg /L	1 g L−1	Khalfaoui et al.67
Coffee residues	~ 77	N/A	N/A	N/A	N/A	Lafi et al.68
White potato peel powder	30.48	~ 2	~ 303 K (30 °C)	50 mg/L	1.0 g	Conrad et al.69
Activated Carbon from Rosemary Root	154.53	~ 3.25	55 °C	50 mg /L	15 mg	Bouzid et al.70
Activated Carbon from Annona squamosa Shell	226.9–435.3 (depending on activation)	N/A	N/A	N/A	N/A	Khan et al.71
Green peel	207.90 mg/g	6.6	10 °C	400 mg/L	0.6 g	Benkouachi et al.72
Chicken bone waste (CBW)	84.84	8	45 °C	300 mg /L	60 mg	This study
Chicken bone waste biochar (CBWB)	138.58	8	50 °C	300 mg /L	50 mg	This study

Thermodynamic parameters of adsorption

The thermodynamic assessment of methyl red adsorption onto CBW and CBWB reveals key insights into spontaneity and energetics based on the values presented from Fig. 9 and Table 4. Across the temperature range examined, the ΔG° values were negative for both adsorbents (CBW: − 0.25 to − 2.33 kJ mol⁻¹; CBWB: − 0.57 to − 2.77 kJ mol⁻¹), signifying the spontaneous nature of the process and suggesting increased favorability at elevated temperatures^11,15,21. The more negative ΔG° values noticed for CBWB indicate a stronger affinity of the biochar toward methyl red. Atmani et al.²⁵ has reported a similar temperature-enhanced adsorption pattern for dye sorption using surface-engineered adsorbents and biochar-based, where increased thermal energy enhanced diffusion and active-site accessibility.

Fig. 9.

Thermodynamics parameters on the adsorption of methyl red dye via chicken bone waste and biochar adsorbents.

Table 4.

Thermodynamics values on the adsorption of methyl red dye via chicken bone waste and biochar adsorbents.

	Chicken bone waste			Chicken bone waste biochar
T	ΔG (kJ/mol)	ΔH (kJ/mol)	ΔS (J/mol·K)	ΔG (kJ/mol)	ΔH (kJ/mol)	ΔS (J/mol·K)
298	− 0.25			− 0.57
303	− 0.44			− 0.95
308	− 0.78			− 1.24
313	− 1.05	14.53	0.051	− 1.86	44.06	0.147
318	− 1.22			− 1.93
323	− 1.86			− 2.04
328	− 2.08			− 2.35
342	− 2.33			− 2.77

The positive ΔH° of 14.53 kJ mol⁻¹ for CBW and 44.06 kJ mol⁻¹ for CBWB infer that adsorption is endothermic^11,73, with CBWB demanding higher energy input to drive dye removal. The ΔH° value for CBW below 20 kJ mol⁻¹ is feature of physisorption dominated by weak van der Waals forces and electrostatic interactions^73,74. On the other side, the higher ΔH° value for CBWB (20–80 kJ mol⁻¹) signify the participation of stronger interactions, usually associated with chemisorption mechanisms like ion exchange and π–π electron donor–acceptor interactions between the carbonized biochar matrix and the aromatic rings of methyl red^73,74. Comparable differences between thermally treated biomass and raw biomass have been reported, highlighting improved adsorption affinity following pyrolysis⁶⁴, as well as enhanced dye adsorption performance at higher temperatures⁵¹.

The ΔS° for both adsorbents (CBW: 0.051 J mol⁻¹ K⁻¹; CBWB: 0.147 J mol⁻¹ K⁻¹) indicate increased randomness at the solid–solution interface, because of the displacement of water molecules and rearrangements of the structure during adsorption^15,62,73. The higher ΔS° value for CBWB suggests greater freedom of dye molecules within its heterogeneous and more porous structure, consistent with Freundlich-type behavior and intraparticle diffusion effects^43,62. Similar entropy-driven adsorption phenomena have been documented for dye uptake using porous biosorbents and biochars⁶². Overall, the thermodynamic investigation supports the isotherm and kinetic reports, indicating that methyl red adsorption onto CBW is mainly physisorption-controlled, however CBWB displays a mixed mechanism with improved energetic favorability because of thermal modification, in alignment with recent environmental thermodynamics report by Akkari et al.⁷⁵.

Desorption and reusability of CBW and CBWB

Desorption studies which assess how well an adsorbent can be regenerated and used across multiple cycles is a key factor in practical and economic viability as depicted in Fig. 10. The effect of different desorbing agents on methyl red by chicken bone waste and chicken bone waste biocher was examined and the results obtained are as illustrated in Table 10a. However, acetic acid exhibited the highest desorption efficiency (70.2% and 76.4% for CBW and CBWB) while preserving the structural integrity of the adsorbent. Thus, acetic acid was selected as the desorbing solvent for regeneration studies. In the current study, CBW and CBWB show decreasing desorption efficiencies over five consecutive cycles with CBW declines from 69.9% in the first cycle to 56.6% in the fifth, while CBWB falls more slowly, from 78.4 to 68.9% (Fig. 10b). This suggests that CBWB is more robust under regeneration than raw CBW. The decline over cycles can be as a result of some dye molecules becoming irreversibly bound in deeper pores or by strong interactions that can’t be fully reversed in regeneration. Also, the initial active functional groups or surface sites may be blocked, degraded, or be lost with repeated chemical or thermal treatments. In a study Eleryan et al.⁷⁶ of Delonix regia-derived biochar-sulfur oxide composite in six cycles of adsorption/desorption with methyl red (and methyl orange) revealed that only modest decreases in adsorption/desorption performance were seen as the composite demonstrated good stability and retained much of its performance after several cycles. For bone char fixed-bed columns treating reactive blue dye BF-5G, Maeda et al.⁷⁷ noted that after multiple adsorption/desorption cycles, some interaction became irreversible, but using ethyl alcohol as regenerating agents helped improve desorption recovery. In this present study, CBWB exhibits relatively better stability with 68.9% desorption remains after the 5th cycle, which is quite good compared with some bone char systems where desorption falls off more sharply. CBW, while starting reasonably well, has more rapid decline, likely due to its less stable matrix and fewer structuring modifications from pyrolysis.

Fig. 10.

(a) Effect of different desorbing solvents, and (b) Desorption of methyl red dye via chicken bone waste and biochar adsorbents.

Proposed adsorption mechanism

The adsorption of methyl red (MR) onto chicken bone waste (CBW) and its biochar derivative (CBWB) can be attributed to a combination of several mechanisms occurring via pore diffusion and surface functional group interactions, electrostatic attraction. FT-IR assessment confirmed the presence of –OH, CO₃²⁻, and PO₄³⁻ groups on the adsorbent surface, which actively participated in MR removal. Shifts and intensity changes in the –OH and PO₄³⁻ bands after sorption process suggest their involvement in surface complexation and ion exchange mechanisms^78–80. For CBWB, the reduction in polar groups after calcination was accompanied by the development of aromatic carbon domains, which promoted π–π interactions with the aromatic structure of MR molecules, thereby enhancing adsorption capacity⁸¹. The schematic illustration of the adsorption mechanism of MR onto CBW and CBWB, indicates the functional moieties of MR (sulfonate –SO₃⁻, azo –N = N–, aromatic rings), the charged surface functional groups (–PO₄³⁻,–O⁻, Ca²⁺, –CO₃²⁻), and their specific interactions such as hydrogen bonding (O–H···N/O), electrostatic attraction (O⁻···N⁺), ion exchange (Ca²⁺···SO₃⁻), and π–π interactions (aromatic C = C domains). Electrostatic effects also played a vital role, as demonstrated by the point of zero charge (pH_pzc) values of CBW and CBWB (5.5–6.2). Below this pH range, the positively charged surface repelled MR molecules, while above it, the negatively charged surface enhanced strong electrostatic attraction, leading to improved adsorption performance³⁸. Isotherm studies further elucidated the mechanism. For instance, it was observed that MR uptake by CBW followed the Langmuir model (Q_max = 84.84 mg/g, R² = 0.996), suggesting monolayer adsorption on relatively homogeneous sites, while CBWB aligned more strongly with the Freundlich model (K_F = 96.67 mg/g, R² = 0.998), indicating heterogeneous multilayer adsorption with higher affinity sites^21,51. Kinetic modeling provided additional insights with CBW fitted the pseudo-first-order model (R² = 0.978–0.998), suggesting adsorption governed mainly by physisorption and van der Waals forces. On the other hands, CBWB data followed the pseudo-second-order model (R² = 0.987–0.997), indicating stronger interactions like chemisorption via π–π donor–acceptor or ion exchange mechanisms^6,60. Intraparticle diffusion contributed to both systems but was not the sole rate-limiting step, as non-zero intercepts were observed in the Weber–Morris plots⁴³. Altogether, the uptake of the dye onto CBW and CBWB is best described as a multi-mechanistic process, involving hydrogen bonding, electrostatic attraction, ion exchange, and π–π interactions. However, it should be noted that these mechanisms are regarded as probable and inferential, rather than conclusively established. Figure 11 illustrate the schematic diagram of methyl red dye adsorption mechanisms by chicken bone waste and biochar adsorbents.

Fig. 11.

Schematic diagram of methyl red dye adsorption mechanisms by chicken bone waste and biochar adsorbents.

Structural evaluations

SEM investigation

Figure 12a represents the SEM image of raw chicken bone waste prior to adsorption. The surface appears compact, dense, and relatively smooth, with fewer visible pores. This morphology indicates a low surface area and limited sorption capacity. After sorption, as shown in Fig. 12b, the surface becomes irregular, with evidence of covering parts of the structure. This is an indicative of adsorbate molecules binding onto the available surface. In contrast, Fig. 12c shows the morphology of chicken bone waste after calcination into biochar. The surface is irregular and porous, with a network of cavities and channels that are clearly visible at the micron scale. The observed porous architecture is as a result of thermal treatment, which removes organic matter and exposes the inorganic fraction. Figure 12d illustrates the biochar after sorption with less pores observed as in (c) which appear to be filled or partially blocked, indicating the uptake of adsorbate molecules. The surface is now smoother compared to the biochar before adsorption, confirming that pollutant molecules have occupied the available pores.

Fig. 12.

SEM image of raw chicken bone waste prior to adsorption (a), after adsorption (b), chicken bone waste biochar prior to adsorption (c), and after adsorption (d).

Thermogravimetric analysis (TGA) of chicken bone waste

The thermogravimetric analysis of the chicken bone waste gave valuable insights into its thermal decomposition behavior and potential applications as a functional adsorbent as shown in Fig. 13. As shown in the TGA profile, an initial slight weight loss occurs below 150 °C, which can be assigned to the release of volatile compounds and physically adsorbed moisture⁸⁰. At the temperature rose around 150 °C and 400 °C, a gradual weight reduction is observed, which corresponds to the decomposition of organic matter like proteins, collagen, and residual lipids^78,80. A more pronounced weight loss is evident in the region of 400 °C and 600 °C, which indicates the degradation of the collagen matrix and carbonate groups associated in the bone structure composed of hydroxyapatite structure (Ca₁₀(PO₄)₆(OH)₂)^48–50. Beyond 600 °C, the curve stabilizes, with minimal weight loss up to 750 °C, suggestting the retention of thermally stable adsorbent^79–81, and that further heating mainly promotes structural rearrangement instead of additional decomposition. Heating to 750 °C thus ensures complete elimination of organic constituents while retaining the thermally stable hydroxyapatite components and combined with the carbonized inorganic framework formed during pyrolysis, which is important for maintaining mechanical integrity and adsorption functionality. Within this temperature, the inorganic phase undergoes partial recrystallization, as established by sharper and more intense hydroxyapatite peaks in the XRD investigation, while simultaneous volatilization of organic matter produces a well-developed porous carbonaceous matrix. During pyrolysis, the thermal decomposition of organic components such as collagen, proteins, and lipids occurs leading to the formation of carbon-rich aromatic domains, which enhance π–π interactions with the aromatic rings of methyl red. Thus, pyrolysis improves functional site accessibility by removing obstructive organic matter, porosity and increasing surface area, improving pore diffusion pathways, and exposing chemically active mineral and carbonaceous sites that collectively contribute to the enhanced adsorption performance of the biochar. These combined effects increase both the effectiveness and availability of functional sites.

Fig. 13.

TGA assessment of raw chicken bone waste.

A clear uniformity is seen among the thermodynamic parameters, TGA results, and the experimentally observed effect of temperature on methyl red uptake. Thermogravimetric results shows that only major mass loss occurring only at elevated temperatures because of the decomposition of residual organic matter. This improved thermal stability reflects the formation of a rigid carbon–mineral framework through pyrolysis, which remains structurally intact under increasing thermal conditions. This structural toughness directly relates with the thermodynamic findings with positive enthalpy change computed for the adsorption process confirming that the process is endothermic, and signifying that higher temperatures promote dye uptake by activating additional adsorption sites and enhancing adsorbate–adsorbent relationship. In addition, the positive entropy change further indicates enhanced randomness at the solid–liquid interface, which is consistent with the disruption of hydration layers and improved molecular mobility at higher temperatures. Experimentally, the adsorption capacity increases with rising temperature, which agrees well with both the thermal stability demonstrated by TGA and the endothermic nature of the process.

BET analysis

The BET analysis compares the nitrogen adsorption behavior of chicken bone waste and chicken bone waste biochar, providing insight into their porosity and surface area as revealed in Fig. 14. Figure (a) indicates the adsorption isotherms of CBW and CBWB as a function of relative pressure (P/P₀). Both materials demonstrated a gradual rise in adsorption with increasing relative pressure, indicating the availability of porous structures capable of gas uptake. Though, CBW consistently shows greater adsorption values than CBWB across the entire relative pressure range. At low relative pressures (P/P₀ < 0.2), the sharp rise in adsorption signifies the presence of micropores in both adsorbents. The total pore volume computed at relative pressures close to unity was approximately 0.045 cm³ g⁻¹ for CBW and 0.078 cm³ g⁻¹ for CBWB, signifying a substantial increase in pore volume after pyrolysis. This affirms that thermal activation significantly improved pore development in the biochar. The better adsorption capacity of CBW in this region suggests a stronger adsorbate–adsorbent interactions or greater availability of accessible micropores compared to CBWB. Figure (b) shows the cumulative pore volume as a function of pore diameter. The materials display pore size distribution predominantly within the range of 1.0–1.8 nm, confirming that microporous dominate their pore structures. This is an indication that the biochar preparation process promotes the development of microporous, which are particularly advantageous for the adsorption of larger molecules such as organic dyes. The cumulative pore volume plot reveals a significant enhancement in porosity after pyrolysis of chicken bone waste. The CBWB sample exhibits a higher cumulative pore volume and a steeper increase across the microporous region compared to CBW, suggesting the development of a well-defined porous structure. This improvement may be attributed to thermal decomposition and volatilization during biochar formation, which generates additional micropores.

Fig. 14.

BET plots of (a) adsorption isotherm analysis and (b) cumulative pore volume as a function of pore diameter.

The BET analysis yielded a specific surface area of approximately 37.3 m²/g for CBW and 76.4 m²/g for CBWB which is an indication that CBWB has a much larger accessible surface for adsorbate interaction, consistent with its observed superior performance in dye and heavy metal adsorption. Côrtes et al.⁷⁹ prepared a carbonaceous material from the pyrolysis of chicken bones reported BET surface area of 18.72 m²/g and pore volume 0.075 cm³/g; also, has activated carbon version with 108.94 m²/g. Yang et al.⁸² used chicken bone to derived mesoporous biochar with lower pyrolysis temperature to give a more polar functional groups and functional behaviour.

FTIR analysis of chicken bone waste and biochar

FT-IR spectra of raw chicken bone waste (CBW) and its derived biochar before and after adsorption are presented in Fig. 15. The prominent functional groups recognized in the FTIR spectrum include –OH, CO₃²⁻, and PO₄³⁻, which are characteristic of hydroxyapatite, the principal inorganic composition of bone⁷⁸. In the raw CBW, a broad absorption band noticed within 3468 and 3575 cm⁻¹ denote –OH stretching vibrations, whereas peaks in the region of 1480–1504 cm⁻¹ and around 1040 cm⁻¹ correspond to CO₃²⁻ and PO₄³⁻ vibrational modes, respectively⁷⁸. These characteristics are in accordance with the mineral structure of bone hydroxyapatite⁸³.

Fig. 15.

FTIR analysis of raw chicken bone waste prior to adsorption (a), chicken bone waste biochar prior to adsorption (b), raw chicken bone waste after adsorption (c), and chicken bone waste biochar after adsorption (d).

After calcination, the biochar spectrum demonstrated a noticeable decline in –OH band intensity, signifying structural reorganization due to dehydration. The characteristic CO₃²⁻ and PO₄³⁻ peaks remain present but show peak broadening and slight shifts, indicating changes in mineral phase composition and crystallinity induced by thermal treatment⁷⁹.

After adsorption, both raw CBW and biochar samples show visible shifts and intensity disparities in the –PO₄³⁻ and OH bands, implying that these functional groups contributed significantly towards the adsorption process via ion-exchange and/or surface complexation mechanisms^79,80,83. New or intensified absorption peaks are noticed at 3475, 1467, and 1055 cm⁻¹ for the raw CBW and at 3459, 1485, and 1065 cm⁻¹ for the biochar, signifying –OH, CO₃²⁻, and PO₄³⁻ stretching respectively, further affirming their participation in adsorbate binding.

More pronounced changes are obvious in the spectral of the post-adsorption FT-IR of CBW (Fig. 14c), most especially the enhancement and emergence of peaks in the ranges of 1720–1750 cm⁻¹ and 1080–1120 cm⁻¹, which are weak or absent in the pristine adsorbent. The peak near 1750 cm⁻¹ corresponds to C = O stretching vibrations of carboxylic groups obtained from the methyl red dye molecule. The improvement in intensity of this band suggests strong interactions between the surface hydroxyl or phosphate groups and dye’s carboxyl functionality through surface complexation or hydrogen bonding^79,80]. Moreover, the intensified band around 1102 cm⁻¹ denotes S = O stretching vibrations of sulfonate (–SO₃⁻) groups in methyl red, overlapping with intrinsic PO₄³⁻ peaks of the bone-derived material^6,78,79. The improvement and slight shift of this peak signify ion-exchange or electrostatic attraction interactions between surface Ca²⁺ ions or protonated functional groups of the adsorbent and negatively charged sulfonate groups^38,39. Literature findings reveals that the major surface functionalities as identified by FT-IR investigation are related to hydroxyl, phosphate and carbonate groups⁸³. Generally, the observed FT-IR spectral differences after adsorption affirms successful immobilization of methyl red onto the CBW and biochar surfaces and support the participation of phosphate, carbonate, and hydroxyl functional groups in the adsorption mechanism. It has been reported that changes in FTIR peak positions and intensities after adsorption is an indication of the involvement of surface functional groups, which play a vital role in uptake processes⁸⁴.

XRD analysis of chicken bone waste and biochar

The X-ray diffraction (XRD) patterns of chicken bone waste and biochar before and after adsorption (Fig. 16) show characteristic crystalline peaks of hydroxyapatite (HA). The main reflections at 2θ ≈ 26° (002), 31.7° (211), 32.9° (300), 39.8° (310), 46.7° (222), 49.5° (213), and 53.2° (004) correspond to standard HA crystallographic planes (JCPDS No. 09–0432)^81,85. The diffraction peaks in pattern (a) are broad and of lower intensity, indicating low crystallinity of the bone mineral. This is a reflection of the natural bone matrix where hydroxyapatite is nanocrystalline and often carbonate-substituted, as found in other biological apatites⁸¹. After calcination, pattern (b) shows sharper and more intense peaks, suggesting increased crystallinity of hydroxyapatite. This could be as a result of the removal of organic matter and partial recrystallization during heat treatment. Thermal treatment typically enhances the order of HA in bone char⁸¹. The spectrum (c) resembles pattern (a) but with subtle peak intensity changes as adsorption may have caused surface modification or a minor reorganization of the HA lattice, though the overall structure remains intact. Slight intensity changes and broadening of peaks suggest weak interaction between the adsorbate and mineral phase. Pattern (d) is also similar to (b) but shows slight shifts and reduced peak intensity, especially at the (211), (300), and (310) planes. These changes may be as a result of adsorbate interaction with active crystalline sites, which could involve the likes of ion exchange or surface complexation with phosphate groups⁸³. These structural effects are consistent with the involvement of the hydroxyapatite phase in the adsorption process; though, specific mechanisms such as ion exchange or surface complexation are inferred from complementary analyses rather than confirmed by XRD alone⁸¹. The XRD comparison between raw chicken bone waste and its biochar exhibits clear structural differences, with the raw material exhibiting broad, low-intensity hydroxyapatite peaks as a result of the presence of an amorphous organic–mineral matrix, and poor crystallinity while the biochar shows more intense and sharper hydroxyapatite peaks, signifying increased crystallinity caused by organic matter elimination during pyrolysis. The reduced broadening and improved peak intensity confirm improved structural ordering and partial recrystallization of the hydroxyapatite phase. After methyl red uptake, the biochar XRD pattern shows slight decline in peak intensity and minor broadening at key hydroxyapatite planes (2θ ≈ 31.7°, 32.9°, and 39.8°), with no formation of new crystalline phases. These alterations suggest that the adsorption process may involve surface-level interactions such as hydrogen bonding with phosphate groups, π–π interactions with carbonized domains, and electrostatic interactions with Ca²⁺ sites, without altering the bulk crystal structure. In all, the XRD results suggests successful dye uptake while conserving the structural integrity and reusability of the biochar.

Fig. 16.

XRD analysis of raw chicken bone waste prior to adsorption (a), chicken bone waste biochar prior to adsorption (b), raw chicken bone waste after adsorption (c), and chicken bone waste biochar after adsorption (d).

Environmental and economic benefits of chicken bone waste–derived biochar

The transformation of chicken bone waste (CBW) into biochar offers notable environmental benefits over conventional carbonaceous material production approaches. Chicken bone waste is produced in huge quantities by the poultry industry and is usually disposed of in landfills or open dumping sites, causing pathogen proliferation, odor problems, and greenhouse gas emissions. Conversion of this waste into biochar diverts it from disposal pathways, decreases environmental pollution, and encourages circular economy principles by converting an otherwise problematic waste into a functional material.

Unlike many activated carbon production pathways that rely on non-renewable precursors like coal or energy- and chemical-intensive activation processes involving strong bases or acids, the biochar preparation method used in this work comprises a single-step thermal treatment without chemical activation. This reduces chemical consumption, lessens secondary pollution, and lowers the environmental footprint associated with adsorbent production. Moreover, the inherent mineral content of CBW, most especially hydroxyapatite, contributes directly to sorption performance, eliminating the need for extensive surface modification.

From an economic viewpoint, chicken bone waste is widely available and an inexpensive raw material, often obtained at little or no cost. The production process only entails only washing, drying, and controlled pyrolysis, making it simpler and possibly more scalable than multistep activation methods used for high-surface-area commercial carbons. The high adsorption capacity obtained, together with the reusability potency demonstrated by the biochar across multiple adsorption–desorption cycles, further boosts its economic attractiveness by dipping material replacement frequency. Potential economic limitations may include energy demand associated with high-temperature pyrolysis and the need for controlled thermal equipment. Nevertheless, these costs may be offset by the absence of chemical reagents, low precursor cost, and long service life of the biochar. In general, the environmental and economic trade-offs favor chicken bone waste–derived biochar as a cost-effective and sustainable alternative to conventional carbonaceous adsorbents for wastewater treatment applications.

Comparison with literature and origin of enhanced adsorption performance

Compared with previously documented bone-derived biochars that rely solely on hydroxyapatite-mediated adsorption, CBWB exhibits a broader interaction spectrum because of the presence of carbonized domains produced during pyrolysis. These domains introduce π-electron-rich surfaces that can easily interact with the aromatic structure of methyl red via π–π stacking, complementing the ion-exchange and the electrostatic interactions provided by Ca²⁺ and PO₄³⁻ sites. This synergistic interaction framework contributes to the enhanced adsorption capacity and faster kinetics noticed in case of CBWB compare with raw CBW and other reported bone-based adsorbents. The adsorption capacities obtained in this work for chicken bone waste (CBW: 84.84 mg/g) and particularly its biochar derivative (CBWB: 138.58 mg/g) are significantly higher than most chicken-bone-based and other biomass-derived adsorbents documented for methyl red removal (Table 3). For instance, Bali cow bone hydrochar produced a capacity of only 7.2 mg/g, biogas plant waste-derived carbons demonstrated 31–113 mg/g depending on activation conditions, while plant-based biochars like fennel seed, moringa shell, and potato peel generally exhibited capacities below 80 mg/g. The superior performance seen in the present study can be ascribed to a combination of thermal treatment strategy, surface chemistry, precursor composition, and adsorption mechanism synergy^32,36.

Firstly, chicken bone waste inherently contains a hybrid inorganic–organic framework of hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) embedded within a carbonaceous matrix^37,81,83. Unlike purely plant-based biochars, this dual-phase structure offers both mineral-based active sites (calcium and phosphate groups) and carbon-based adsorption domains, aiding multiple interaction pathways with methyl red molecules. Previous reports on chicken bone adsorbents relied either on mild chemical activation or low-temperature hydrothermal treatment, which impeded pore development and site accessibility, thus limiting adsorption capacity.

Additionally, the average-temperature pyrolysis employed in this study (750 °C) plays a decisive role in improving adsorption performance. Thermal treatment at this temperature effectively eliminate organic residues, increases crystallinity, and produces a well-developed micro–mesoporous structure, as confirmed by SEM and BET analyses. The resulting surface area of CBWB (76.4 m²/g) is significantly higher than that of raw CBW (37.3 m²/g) and surpasses values reported for some bone-derived biochars produced at lower temperatures. This improved porosity enhances intraparticle diffusion and permits dye molecules to access deeper adsorption sites, which contributes directly to the higher equilibrium uptake.

Furthermore, surface chemistry evolution during pyrolysis greatly enhances adsorption affinity^32,36. While raw CBW adsorption is governed by weak van der Waals and electrostatic interactions, CBWB demonstrated additional π–π interactions between the aromatic rings of methyl red and aromatic domains of the carbonized biochar, as supported by kinetic and thermodynamic results^6,60,73. This is evidence from the transition from pseudo-first-order kinetics (physisorption-dominated mechanisms) for CBW to pseudo-second-order kinetics (chemisorption-dominated mechanisms) for CBWB, indicating stronger and more specific interactions.

Finally, regeneration property also differentiates this adsorbent from previously reported systems. CBWB retained nearly 69% of its adsorption efficiency after five cycles, surpassing many literature reports where bone-based adsorbents suffer rapid performance decline because of pore blockage or structural degradation. The thermally stabilized hydroxyapatite–carbon framework of CBWB facilitated this durability.

Conclusion

This study establishes innovative and sustainable approach for wastewater remediation through the valorization of chicken bone waste into an efficient biochar adsorbent for methyl red dye uptake. The key innovation of this work lies in the direct comparison between raw chicken bone waste and its biochar derivative under similar experimental conditions, which clearly explains the impact of thermal valorization on adsorption performance without the need for chemical activation or surface modification.

Physicochemical investigations revealed that pyrolysis at 750 °C greatly improved the structural features of the material, increasing the surface area from 37.3 m²/g for CBW to 76.4 m²/g for CBWB. These structural enhancements translated into markedly higher adsorption performance. The maximum monolayer adsorption capacities found from Langmuir isotherm analysis were 84.84 mg/g for CBW and 138.58 mg/g for CBWB, signifying approximately a 63% improvement in adsorption capacity because of biochar formation.

Adsorption kinetics revealed that MR uptake by CBW followed the pseudo-first-order model, signifying a physisorption-dominated mechanism, whereas CBWB followed the pseudo-second-order model, demonstrating stronger and mixed adsorption interactions. Equilibrium studies further confirmed that CBW adsorption was best described by the Langmuir model (R² = 0.996), while CBWB adsorption followed the Freundlich model (R² = 0.998), reflecting the heterogeneous surface nature of the biochar. Thermodynamic analysis confirmed that adsorption was spontaneous and endothermic for both adsorbents, with ΔH° values of 14.53 kJ/mol for CBW and 44.06 kJ/mol for CBWB.

When compared with previously reported methyl red adsorbents, including agricultural residues, plant-based biochars, and bone-derived hydrochars, the chicken bone waste biochar prepared in this study demonstrated superior adsorption capacity without chemical activation, highlighting its practical and environmental advantages. Additionally, regeneration investigation demonstrated that CBWB retained approximately 69% of its initial adsorption efficiency after five adsorption–desorption cycles, signifying good reusability and operational stability.

In all, this study establishes chicken bone waste-derived biochar as a high-performance, low-cost, and environmentally benign adsorbent for dye-contaminated wastewater. The findings not only contribute to the growing body of knowledge on methyl red removal but also provide a viable pathway for transforming poultry waste into value-added materials, supporting sustainable water treatment technologies.

To obtain more definitive insights into the binding interactions and surface chemical states, future studies employing advanced characterization techniques such as X-ray photoelectron spectroscopy (XPS) are recommended.

Author contributions

Ibtehaj F. Alshdoukhi: Contributed resources and software, Khairia Mohammed Al-Ahmary and Saedah R. Al-Mhyawi: Performed the data analysis, contributed software and vetted the first draft, Jawza Sh Alnawmasi: Contributed software and performed characterization, ‏Zahrah Alqahtani: Contributed reagents; Performed data analysis and vetted the second draft, Hamad AlMohamadi: Performed data analysis; and Contributed software, and Jawaher Saud Alrashood Contributed finance and Contributed software, Oladejo Emmanuel Kola,: Conceived the experiments; Performed the experiments; and wrote the first draft paper.

Funding

This work was financed by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2026R581), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

Data availability

The datasets used and/or analyzed in this study are available from the corresponding author upon reasonable request.

Declarations

Competing interests

The authors declare that they have no competing interests.