# Synthesis, characterization, and performance evaluation of a new innovative hydroxyapatite-based biocomposite for wastewater treatment application

Sawsen Abed, Wahiba Lemlikchi, Lokmane Abddelouahed, Hocine Grabi, Noureddine Youdif, Khadidja Adjir, Zineb Boutamine

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Sawsen Abed, Wahiba Lemlikchi, Lokmane Abddelouahed, Hocine Grabi, Noureddine Youdif, et al. Synthesis, characterization, and performance evaluation of a new innovative hydroxyapatite-based biocomposite for wastewater treatment application. Separation and Purification Technology, 2026, 394, pp.137435. 10.1016/j.seppur.2026.137435. hal-05536819

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# Synthesis, characterization, and performance evaluation of a new innovative hydroxyapatite-based biocomposite for wastewater treatment application

Sawsen Abed a,b, Wahiba Lemlikchi a,c, Lokmane Abddelouahed b,*, Hocine Grabi a,d, Noureddine Youdif d,e, Khadidja Adjir d, Zineb Boutamine f

## ARTICLE INFO

Editor name: Dr. Harvey Yi Huang

**Keywords:**

Hydroxyapatite

Biocomposite

Biomass

Brushite

Surface characterization

## ABSTRACT

This study illustrates the synthesis and comprehensive characterization of a newly developed Hydroxyapatite-biomass-based biocomposite derived from synthesized hydroxyapatite and biomass, specifically Banana peel (BN). The biocomposite was prepared by integrating hydroxyapatite with Banana peel biomass, promoting the in-situ formation of a brushite calcium phosphate phase within the lignocellulosic matrix, leading to a reactive organo-mineral interface. Both the raw biomaterial and the resulting biocomposite were thoroughly characterized using surface and spectroscopic techniques. Such as Fourier transform infrared spectroscopy FTIR, X-ray diffraction XRD, scanning electron microscopy SEM, elemental analysis CHNS, energy dispersive spectroscopy EDX, thermogravimetric and differential analysis TG-DTA, and Nitrogen-desorption isotherms BET. Moreover, molecular interactions in the biocomposite system were studied using density functional theory DFT to identify the functional groups involved in the formation of the HAp-BN material. The synthesized HAp-BN biocomposite prepared was evaluated and compared with hydroxyapatite (HAp) and biomass (BN) for contaminant removal. Adsorption experiments demonstrate that the HAp-biomass biocomposite achieved significantly higher removal efficiency (96%) for Direct Yellow 177 GLE compared to hydroxyapatite (67%) and Banana peel biomass (36%) under the same operating conditions. The enhanced performance is attributed to the synergistic effect of the brushite phase and the oxygen-containing functional groups of the biomass, facilitating surface complexation, hydrogen bonding, and electrostatic interactions with the dye molecules.

## 1. Introduction

Recently, biocomposites combining inorganic and organic materials have garnered considerable attention for wastewater purification applications due to their synergistic properties, such as enhanced adsorption capacity, structural stability, and environmental compatibility [1,2].

The excellent performance of hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂), a calcium phosphate-based biomaterial, has made it an extremely promising option for treating environmental pollution. Hydroxyapatite can be

extremely useful in the field of pollution control, due to its structure and attractive properties, it possesses excellent capacity of adsorption, ion-exchange, acid-bases, and thermal stability characteristics [3–6].

Hydroxyapatite can be synthesized from natural sources such as animal bones, eggshells and fish scales, making it relatively inexpensive and available in large quantities for industrial applications. It is also known for its ability to be easily regenerated after adsorption of pollutants, making it a durable and economical option for the treatment of pollutants [7–10].

Generally, hydroxyapatite is a versatile and effective adsorbent for various pollutants, making it a valuable element in the management of

<footnote>a Laboratory of Applied Chemistry and Chemical Engineering, UMMTO, Tizi-Ouzou, Algeria 1, 16000, Algiers Center, Algeria</footnote>

<footnote>b INSA Rouen Normandie, University Rouen Normandie, Normandie University, LSPC UR 4704, Laboratory of Chemical Process Safety, F-76000 Rouen, France</footnote>

<footnote>c Laboratory of Storage and Valorization of Renewable Energies, USTHB, Bab Ezzouar, Algeria</footnote>

<footnote>d Laboratory of Thermodynamics and Molecular Modeling, Faculty of Chemistry, USTHB, Algiers, Algeria</footnote>

<footnote>e Laboratoire de Chimie Théorique de Bio- et Nanosystèmes (LCTBN), Faculty of Exact Sciences, University Djillali Liabes of Sidi Bel Abbès, Sidi Bel Abbès, Algeria</footnote>

<footnote>f Laboratory of Process Engineering for Sustainable Development and Health Products, Department of Process Engineering, National Polytechnic School of Constantine, BP 75, A, Nouvelle Ville RP, Constantine, Algeria</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: lokmane.abdelouahed@insa-rouen.fr (L. Abddelouahed).</footnote>
Abbreviations

HAp

Hydroxyapatite

BN

Banana Peel (biomass)

HAp-BN

Hydroxyapatite-Banana Peel biocomposite

FTIR

Fourier transform infrared spectroscopy

XRD

X-ray diffraction

SEM

Scanning Electron microscopy

CHNS

Carbon, hydrogen, nitrogen, and sulfur (elemental analysis)

EDX / EDS

Energy dispersive X-ray spectroscopy

TG-DTA

Thermogravimetric and differential thermal analysis

BET

Brunauer–Emmett–teller (surface area measurement)

Rpm

Revolutions per minute

°C

Degrees Celsius

Mg·L⁻¹

Milligrams per liter

pH

Hydrogen ion concentration (potential of hydrogen)

DCPD

Dicalcium phosphate Dihydrate (Brushite)

Ca(HPO₄)·2H₂O

Chemical formula of Brushite

Ca(OH)₂

Calcium hydroxide

Mm

Micrometer (10⁻⁶ m)

Nm

Nanometer (10⁻⁹ m)

Å

Angstrom (10⁻¹⁰ m)

kV

Kilovolt

pH

Initial pH

JCPDS

Joint committee on powder diffraction standards

D

Crystallite size (Scherrer equation context)

FWHM

Full width at half maximum

λ

X-ray wavelength

θ (theta)

Diffraction angle

PO₄³⁻

Phosphate ion

HPO₄²⁻

Hydrogen phosphate ion

CO₃²⁻

Carbonate ion

OH⁻

Hydroxide ion

C=O

Carbonyl group

N-H

Amine group

C-H

Alkyl group

-COOH

Carboxylic acid group

-COO⁻

Carboxylate group (ionized form of COOH)

O-H

Hydroxyl group (alcohol, phenol, water)

C=O

Carbon-carbon double bond (aromatic, lignin)

C-O

Carbon-oxygen bond (esters, ethers)

P-O

Phosphate vibration

Ca/P

Calcium to phosphorus molar ratio

C

Carbon

H

Hydrogen

N

Nitrogen

O

Oxygen

Na

Sodium

Mg

Magnesium

Al

Aluminum

P

Phosphorus

S

Sulfur

K

Potassium

Ca

Calcium

%mass

Mass percentage (%)

%atom

Atomic percentage (%)

BBOT

2,5-Bis(5-tert-butylbenzoxazol-2-yl)thiophene (CHNS calibration standard)

FLASH 2000

Elemental analyzer instrument (Thermo-fisher)

EAGER 300

Software for CHNS data processing

Tin pan

Tin crucible (sample holder for CHNS)

TG / TGA

Thermogravimetric analysis

TA instruments

Q500 Model name of the thermal analyzer

DTA

Differential thermal analysis

mL/min

Milliliters per minute (gas flow rate)

N₂

Nitrogen (gas used for purge)

TA instruments

Q500 Thermal analyzer model (TGA)

ASAP 2020 M

Accelerated surface area and Porosimetry analyzer (model)

P/P₀

Relative pressure (ratio of partial pressure to saturated vapor pressure)

cm³/g

Cubic centimeters per gram (pore volume unit)

m²·g⁻¹

Square meters per gram (specific surface area)

K

Kelvin (absolute temperature unit)

BioRender

Scientific illustration software

Gaussian

Quantum chemistry calculation software

Ca²⁺

Calcium ion

-OH

Hydroxyl group

-C-O-C⁻

Ether oxygen atoms

BJH

Barrett-Joyner-Halenda method (pore size distribution analysis)

NLDFT

Non-local density functional theory (advanced pore analysis)

DFT

Density functional theory (quantum chemical method)

HOMO

Highest occupied molecular orbital

LUMO

Lowest unoccupied molecular orbital

NPA

Natural population analysis (atomic charge analysis)

Fk⁺ (f⁺)

Fukui function for nucleophilic attack

Fk⁻ (f⁻)

Fukui function for electrophilic attack

B3LYP

Becke, 3-parameter, Lee-Yang-Parr hybrid functional (DFT)

6-311 + G(d,p)

Quantum chemistry basis set

C₁

Molecular symmetry group (no symmetry)

N

Number of electrons in target molecules

Qk(N + 1)

Atomic charges for anion, neutral, and cation species

Qk(N + 1)

Atomic charge of atom k in the anion (molecule with N + 1 electrons)

Qk(N - 1)

Atomic charge of atom k in the cation (molecule with N - 1 electrons)

Qk(N)

Atomic charge of atom k in the neutral molecule (with N electrons)

O3, O23,...

Names/labels of oxygen atoms in the cellulose molecule (specific numbering)

pollution control and water treatment [11].

Banana peel was selected as an organic precursor due to its abundance, low cost, and sustainability, making it an environmentally friendly material. As a common and readily available post-harvest waste, its valorization for pollutant adsorption contributes to environmental protection by reducing agricultural residues and promoting a more sustainable approach to wastewater treatment. Chemically, Banana peel is mainly composed of polysaccharide fractions, including cellulose (18–59%), hemicellulose (17–20%), and pectin (10–20%),

along with other low molecular weight compounds [12–14].

These lignocellulosic components contain abundant oxygen-containing functional groups, such as hydroxyl (–OH), carboxyl (–COOH), and phenolic groups, which can bind dye molecules and metal ions through electrostatic interactions, hydrogen bonding, and surface complexation. Consequently, the effectiveness of Banana peel as a biosorbent for various pollutants has been widely reported in the literature, particularly in applications related to bioremediation and wastewater treatment [15].
<figure><img src="image_3.png" /><figcaption>Fig. 1. SEM images of the studied materials: (a-c) Banana peel (BN), (d-f) synthetic hydroxyapatite (HAp), and (i-k) HAp-BN biocomposite. Images were acquired using a ANA 89 GEOL microscope at 10–20 kV in secondary electron (SE) mode. The magnifications are indicated on each micrograph.</figcaption></figure>

This study aims to develop a cost-effective depollution strategy through the synthesis of a natural, sustainable, and low-cost bio-composite based on hydroxyapatite derived from natural precursors and Banana peel biomass. The resulting materials were thoroughly characterized using a range of analytical techniques and subsequently applied to the treatment of real contamination sources, particularly effluents from metallurgical, textile, and pharmaceutical industries. The bio-composites exhibited high adsorption capacities for dye removal, high-lighting their potential for wastewater treatment applications.

The originality of this work lies in the integration of hydroxyapatite (HAp) and Banana peel biomass into a hybrid biocomposite in which brushite (DCPD) forms in situ on the organic substrate, leading to the development of a reactive organo-mineral interface. This structure, characterized by the in-situ formation of a brushite-rich calcium phosphate phase within the lignocellulosic matrix, provides multiple active sites and promotes effective interactions between mineral phases and biomass functional groups, resulting in superior adsorption performance compared to hydroxyapatite or biomass used individually.

## 2. Material and methods

### 2.1. HAp-BN biocomposite synthesis

The HAp-BN biocomposite was synthesized by combining equal masses of natural hydroxyapatite and biomass, the hydroxyapatite was previously extracted and purified from calcined duck bones, ensuring a stoichiometric composition close to Ca₁₀(PO₄)₆(OH)₂, with the Ca/P ratio confirmed by elemental analysis and X-ray diffraction (XRD) results. To ensure reproducibility, the synthesis of the HAp-BN composite was repeated several times under identical experimental conditions.

For the biocomposite preparation, 1 g of HAp powder was mixed with 1 g of finely ground biomass. The mixture was suspended in an aqueous solution adjusted to a pH above 6 using sodium hydroxide (NaOH, ≥98%, Sigma) and hydrochloric acid (HCl, 37%, Sigma) to

promote recrystallization and ensure homogeneous interaction between the mineral and organic phases. The suspension was magnetically stirred at 1200 rpm for 6 h at room temperature. The suspension was filtered and the solid obtained was dried in a ventilated oven at 105 °C for 24 h to remove residual moisture and ensure the stability of the material. The resulting biocomposite was characterized by using the previously mentioned analytical techniques to assess its structural, morphological, and surface properties.

All chemical wastes and residues generated during the synthesis were collected and disposed of according to local safety regulations and standard laboratory environmental protocols, ensuring minimal environmental impact.

### 2.2. Characterization of the materials

The HAp-BN biocomposite, along with its individual components (hydroxyapatite and banana peel biomass), was thoroughly characterized using a series of complementary physicochemical techniques to investigate their structural, morphological, thermal, elemental, and surface properties.

Fourier Transform Infrared (FTIR) spectroscopy was performed using a Bruker TGA-IR spectrometer. The powdered samples were homogeneously mixed with spectroscopic-grade KBr and directly introduced into the sample holder for analysis. The spectra were recorded in the 4000–400 cm⁻¹ range to identify functional groups and to investigate possible interactions between the mineral phase and the biomass.

Crystalline phases were analyzed by X-ray diffraction (XRD) using a D5000 Bruker diffractometer with cobalt radiation (40 kV, 40 mA). Diffraction patterns were collected over a 2θ range of 5°-70°, and the obtained peaks were indexed by comparison with standard JCPDS files. Crystallite sizes were estimated using the Debye-Scherrer equation applied to the most intense diffraction peaks.

Surface morphology and microstructural features were examined by scanning electron microscopy (SEM) using an ANA 89 GEOL microscope
<figure><img src="image_4.png" /><figcaption>Fig. 2. XRD patterns of (a) Banana peel (BN), (b) synthetic hydroxyapatite (HAp), and (c) HAp-BN biocomposite. Peaks were indexed according to JCPDS standards: HAp (#01-086-1199), brushite Ca(HPO₄)·2H₂O (#01-072-1240), and Ca(OH)₂ (#00-050-0008). The crystallite sizes were estimated using the Debye-Scherrer equation.</figcaption></figure>

operating at an accelerating voltage of 10–20 kV in secondary electron (SE) mode. Samples were dried for 24 h and mounted on carbon tape without conductive coating. Elemental composition and Ca/P ratios were determined using energy-dispersive X-ray spectroscopy (EDS) coupled to the SEM.

Elemental (CHNS) analysis was performed using a Thermo Fisher FLASH 2000 elemental analyzer with EAGER 300 software to determine the carbon, hydrogen, nitrogen, and sulfur contents of the biomass and the biocomposite.

Thermal stability and decomposition behavior were investigated by thermogravimetric and differential thermal analysis (TG-DTA) using a TA Instruments Q500 apparatus. Measurements were conducted under a nitrogen atmosphere from 30 °C to 800 °C at a heating rate of 5 °C/min.

Specific surface area and pore structure were determined by nitrogen

adsorption-desorption measurements at 77 K using a BELSORP MAX II analyzer (Microtrac MRB). Prior to analysis, samples were degassed under vacuum at 120 °C. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method, while pore size distribution was derived from the desorption branch of the isotherm.

Finally, Density Functional Theory (DFT) calculations were performed using Gaussian 16 at the B3LYP/6-311++G(d,p) level to investigate the electronic structure and reactivity of cellulose as a representative component of banana peel biomass. HOMO-LUMO distributions and Fukui functions were analyzed to provide molecular-level insight into adsorption mechanisms.

<figure><img src="image_5.png" /><figcaption>Fig. 3. X-ray diffraction (XRD) pattern of chemically synthesized hydroxyapatite, the peaks were indexed according to the JCPDS standard (#01-086-1199). Crystallite sizes were estimated using the Debye-Scherrer equation.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 4. X-ray diffraction (XRD) pattern of the chemically synthesized HAp-BN biocomposite. The identified peaks correspond to brushite (CaHPO₄·2H₂O, JCPDS #01-072-1240) and calcium hydroxide (Ca(OH)₂, JCPDS #00-050-0008). Crystallite sizes were estimated using the Debye-Scherrer equation.</figcaption></figure>

# 3. Results and discussion

## 3.1. Surface morphology and composition

Scanning electron microscopy (SEM) was used to determine the surface morphology and local composition of our materials: Banana peel, Hydroxyapatite, and HAp-BN biocomposite. Samples were dried for 24 h and mounted on carbon tape without additional coating. The magnifications are indicated directly on the images. Elemental composition was analyzed via EDS.

As illustrated in the SEM images (Fig. 1a-c), the Banana peel particles appear as irregular layers, with sizes ranging from 400 µm, and magnifications of 50 µm, 20 µm, and 5 µm for (a), (b), and (c), respectively. In contrast, the synthetic hydroxyapatite (HAp) observed in Fig. 1 (d-f) shows nanoparticles with diameters ranging from 250 to 450 nm, at magnifications of 70 µm, 20 µm, and 3 µm.

As shown in Fig. 1(i-k) for the biocomposite, brushite was homogeneously formed and deposited on biomass fibers. Depending on the pH, DCPD morphology appears as plate-like or needle-like structures ranging from 2 to 6 µm along the (020) crystallographic direction. The brushite displays snowball-like flower structures with approximate sizes of 4-7 µm. The magnifications for the biocomposite images are 50 µm, 10 µm, and 2 µm for (i), (j), and (k), respectively. [16,17]

## 3.2. Biomaterials structure and crystallinity

Synthesized biocomposite, Hydroxyapatite and bioadsorbant were characterized by XRD spectra and matched with the standard JCPDS data as shown in Figs. 2-4.

Fig. 2(a) shows X-ray diffractograms of a Banana peel. The sample was available in semi-crystalline form and tended toward an amorphous phase. While lignin and hemicellulose always have an amorphous structure, cellulose is one of the lignocellulosic compounds that can be both amorphous and crystalline [18]. The diffraction patterns of biomass revealed two peaks at diffraction angles 2θ of 16° and 22°, assigned to the cellulose crystallographic planes I₁₀₁ and I₀₀₂,

respectively. Additionally, the broad peak at 2θ = 22° and reveal the amorphous nature of Banana peel [19].

According to XRD characterization, the crystalline diffraction peaks of the synthesized Hydroxyapatite are confirmed as illustrated in Fig. 2 (b) and matched with the standard JCPDS (#01-086-1199) data corresponding to Fig. 3. The highest intensity HAp characteristic peak was identified at 2θ value of 31.6°, corresponding to (211) plane of synthesized HAp [20]. Similar well intense characteristic peaks such as (002), (210), (112), (130), (222), (213), (004) corresponding to 25.8°, 28.9°, 32.2°, 39.8°, 46.7°, 49.5°, and 53.2° respectively (2θ diffraction angle) were identified for HAp [21].

It was clearly observed that the biocomposite HAp-BN obtained from Banana peel fibers and hydroxyapatite is partially amorphous and crystalline Fig. 3(c). According to XRD characterization, the peaks observed on Fig. 4 show a crystallinity of a brushite Ca(HPO₄), 2H₂O formed and which were confirmed and correspond to JCPDS standard data (01-072-1240). other peaks were attributed to calcium hydroxide Ca(OH)₂ corresponding to JCPDS standard data (00-050-0008). Intense characteristic pick was identified at 2θ value of 11.65°, corresponding to (020) plane of synthesized brushite Ca(HPO₄), 2H₂O. Characteristically intense peaks such as (-121), (-141), (121), (150), (200), (-152), (240), (062) corresponding to 20.94°, 29.29°, 30.54°, 34.17°, 34.43°, 41.58°, 42.06° and 48.05° respectively were identified for brushite syn Ca(HPO₄), 2H₂O. compared with JCPDS (#01-072-1240) and JCPDS (#00-050-0008). Brushite crystallizes in the monoclinic system (space group I2/a) with crystallographic parameters: a = 5.8120 Å, b = 15.180 Å, c = 6.2390 Å, α = γ = 90° and β = 116.43°. The pattern obtained on the prepared material also reveals the presence of other peaks identified at 2θ values of 31.86°, 39.06°, 45.47°, 26.25°, 28.90° and 46.96°, corresponding to calcium hydroxide Ca(OH)₂ [22,23]. The average crystallite size of the HAp syn and brushite nanoparticles of biocomposite were calculated by Debye Scherrer's equation as follows:

$$D = 0.9\lambda/\beta\cos\theta$$

where D signifies crystallite size, β represents full width at half maximum (FWHM) of the highest intensity peak corresponding to (211)

<table><caption>**Table. 1**
Average crystallite size of synthesized materials calculated using the Debye-Scherrer method.</caption><thead><tr><th>Sample</th><th>Identified Phase</th><th>2θ (°)</th><th>(hkl) plane</th><th>Method</th><th>Crystallite Size (nm)</th></tr></thead><tbody><tr><td>HAp</td><td>Hydroxyapatite</td><td>31.6</td><td>(211)</td><td>Debye-Scherrer</td><td>99</td></tr><tr><td>HAp-BN biocomposite</td><td>Brushite Ca(HPO₄)·2H₂O</td><td>11.65</td><td>(020)</td><td>Debye-Scherrer</td><td>94</td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>Fig. 5. FTIR spectra of (a) Banana peel biomass (BN), (b) synthesized hydroxyapatite (HAp), and (c) HAp-BN biocomposite.</figcaption></figure>

<table><caption>**Table. 2**
Summary of FTIR Spectral Bands and Functional Group Assignments for Banana peel (BN), Hydroxyapatite (HAp), and biocomposite (HAp-BN).</caption><thead><tr><th>Biomaterials</th><th>Wavenumber (cm<sup>-1</sup>)</th><th>Functional group / assignment</th><th>Interpretation</th><th>References</th></tr></thead><tbody><tr><td rowspan="5">Banana Peel (BN)</td><td>3405</td><td>O-H stretching (hydroxyl groups of alcohols, phenols), N-H stretching (amines)</td><td>Broad and intense peak indicating presence of hydroxyl and amine groups typical of biomass</td><td>[22,23]</td></tr><tr><td>2960, 2800, 3000</td><td>C-H stretching (-CH<sub>2</sub>-, -CH<sub>3</sub> groups)</td><td>Characteristic vibrations from cellulose, hemicellulose and lignin</td><td>[22-24]</td></tr><tr><td>1733</td><td>C=O stretching (aldehydes or carboxylic acids)</td><td>Carbonyl groups from cellulose and hemicellulose components</td><td>[22,23]</td></tr><tr><td>1633</td><td>-COO symmetric stretching, N-H bending</td><td>Carboxylate groups and amine bending vibrations</td><td>[22,23]</td></tr><tr><td>1524, 1442</td><td>C=C stretching vibrations of aromatic rings(lignin)</td><td>Indicative of lignin structure</td><td>[22,23]</td></tr><tr><td rowspan="5">Hydroxyapatite (HAp)</td><td>1377, 1222, 1072</td><td>C-H bending (crystalline cellulose), C-H bending (hemicellulose, lignin), C-O stretching (ester, ether)</td><td>Reflects the complex polymeric structure of biomass</td><td>[25,26]</td></tr><tr><td>800-600</td><td>N-H bending, Amine groups</td><td>Weak bands attributed to amines</td><td>[22-24]</td></tr><tr><td>3567, 3400, 631</td><td>Hydroxyl group (O-H) stretching</td><td>Hydroxyl groups in hydroxyapatite structure</td><td>[27,28]</td></tr><tr><td>1645</td><td>Bending vibration of adsorbed water molecules</td><td>Adsorbed moisture presence</td><td>[28]</td></tr><tr><td>1385</td><td>C-O stretching (carbonate group CO<sub>3</sub><sup>2-</sup> impurities)</td><td>Carbonate impurities adsorbed from air</td><td>[27,28]</td></tr><tr><td rowspan="10">Biocomposite (HAp-BN)</td><td>1000-1100</td><td>P-O stretching (phosphate group PO<sub>4</sub><sup>3-</sup>)</td><td>Broad band typical of phosphate group vibrations in hydroxyapatite</td><td>[27]</td></tr><tr><td>472, 571, 601, 964, 1052, 1096</td><td>P-O stretching and bending vibrations (PO<sub>4</sub><sup>3-</sup>)</td><td>Confirmed presence of phosphate presence, Brushite</td><td>[27]</td></tr><tr><td>874</td><td>HPO<sub>4</sub><sup>2-</sup> vibrations</td><td>Acid phosphate group vibrations</td><td>[27,28]</td></tr><tr><td>3572, 3492, 3285, 3162</td><td>O-H vibrational modes of water in brushite (DCPD)</td><td>Peaks characteristic of brushite hydrated phase</td><td>[29,30]</td></tr><tr><td>1223, 1645</td><td>O-H bending modes (surplus water)</td><td>Water absorption bands related to biocomposite synthesis</td><td>[29,30]</td></tr><tr><td>563, 580, 1057, 1123, 1132</td><td>P-O stretching, PO<sub>4</sub><sup>3-</sup> ion</td><td>Confirmation of phosphate ions in DCPD phase,</td><td>[29,30]</td></tr><tr><td>901</td><td>HPO<sub>4</sub><sup>2-</sup> vibration</td><td>Confirms brushite formation</td><td>[29,30]</td></tr><tr><td>3200-3800</td><td>O-H, N-H stretching</td><td>Reduction indicates interaction/bonding between DCPD and biomass hydroxyl groups</td><td>[30,31]</td></tr></tbody></table>

plane, λ signifies the diffraction wavelength (CoKa1, λ =1.78897Å), whereas θ is the diffraction angle. The calculated average crystallite size of the synthetic HAp and biocomposite samples were 99 nm and 94 nm respectively. These values are in good agreement with SEM result. The average crystallite sizes of the synthesized hydroxyapatite and the HAp-BN biocomposite were estimated using the Debye-Scherrer equation based on the most intense diffraction peaks. The obtained values are summarized in Table.1.

Notably, the in-situ formation of brushite from hydroxyapatite and biomass provides a more open crystalline lattice, rich in hydroxyl groups

(-OH) as well as HPO<sub>4</sub><sup>2-</sup> and Ca<sup>2+</sup> ions, which are more accessible for potential interactions. When combined with the functional groups of the biomass (-OH, -COOH, -C-O-C-), this configuration increases the overall density of active sites, potentially enhancing the adsorption performance compared to pure HAp.

### 3.3. FTIR analysis of the HAp-BN biocomposite

FTIR spectroscopy was employed to characterize the structural changes occurring during the synthesis of the HAp-BN biocomposite.
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Structure representations of Brushite: x-axis view, z-axis view and y-axis view. Calcium ions are represented in green, phosphate groups in orange, oxygen atoms in red, and hydrogen atoms in white. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

The FTIR spectra of synthesized hydroxyapatite (HAp), biomass (BN), and the HAp-BN biocomposite is shown in Fig. 5, and the main absorption bands are summarized in Table 2.

Different peaks were observed in the FT-IR spectra of the biomass spectrum Fig. 5(a), showing the presence of several functional groups of hydroxyl, carboxyl, amine, and aromatic groups. These bands are mainly attributed to structural components such as cellulose, hemicellulose, and lignin, which are abundant in the biomass. The dominant presence of hydroxyl and carboxylic groups, potentially associated with cellulose, while the amine functionalities may originate from hemicellulose [22–26]. These assignments are supported by literature reports describing the characteristic vibrations of lignocellulosic biomass components.

The typical peaks of HAp are shown on Fig. 5(b), confirming the presence of phosphate groups (PO₄³⁻ and HPO₄²⁻), carbonate impurities, adsorbed water, and hydroxyl groups [27]. The assignments of these peaks are consistent with prior studies on synthetic hydroxyapatite. The presence of C—O band appeared around 1385 cm⁻¹ corresponded to carbonate group CO₃²⁻ impurities which were adsorbed from the air onto the surface of HAp [28]. The band at 1645 cm⁻¹ belongs to the bending vibration of adsorbed water molecules.

As shown in Fig. 5(c), New peaks appear in the biocomposite's FTIR spectrum, characteristic of brushite (DCPD), The peaks appearing as a doublet at 3572, 3492, 3285 and 3162 cm⁻¹ are attributed to the O—H vibrational modes of water in brushite, bending modes associated with O—H bonding were observed as a shoulder at 1223 cm⁻¹ and as a broad band at 1645 cm⁻¹, attributed to the surplus water in the base

formulation. Bands at 563, 580, 1057, 1123 and 1132 cm⁻¹ had been identified to PO₄³⁻ ions, while bands at 1649 and 3000–3428 cm⁻¹ were due to absorbing water. HPO₄²⁻ ions were identified as the source of an absorption band at 901 cm⁻¹, confirming the formation of brushite [31,32].

Furthermore, there is a significant reduction in the intensity of the OH, C=O, N—H, C—H and -COOH peaks at 3200–3800, 1727, 1500, 2906 and 1633 cm⁻¹ respectively. This decrease indicates interactions between the functional groups of the biomass and the mineral phase, such as hydrogen bonding and electrostatic interactions, consistent with previously reported mechanisms for hydroxyapatite–biomass bio-composites [31,32].

Nucleation and growth are the two series of reactions involved in the creation of the HAp-BN biocomposite. Dipolar ionic interactions between calcium ions and carboxylate groups on the BN surface trigger nucleation [33]. While hydrogen bonding occurs between brushite and the hydroxyl groups of biomass, both intramolecularly and intermolecularly.

The negative charges of the carboxylate groups are readily associated with the positive charges of the calcium ions. Another bond is the hydrogen bond between brushite and the hydroxyl groups of bio-materials, which occur both intramolecularly and intermolecularly [31,32].

Fig. 6 shows the structure and crystalline unit cell of dicalcium phosphate dihydrate, or Ca(HPO₄)₂H₂O (DCPD), which crystallizes in the monoclinic space group I₂, and the crystals are composed of parallel-oriented, corrugated brushite sheets with a CaHPO₄ composition [34].
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** EDX spectrum and elemental analysis of (a) Banana peel (BN), (b) synthetic hydroxyapatite (HAp), and (c) HAp-BN biocomposite.</figcaption></figure>

Its structure consists of parallel calcium layers separated by HPO₄ groups positioned between the metal ions [35]. Water molecules act as dividers between these calcium phosphate layers, which lie parallel to each other in the a,c (020) plane [36]. The molecular and crystalline structures were modeled and optimized using Gaussian software, allowing accurate visualization of the atomic arrangements and spatial geometry shown in the x-, y-, and z-axis projections of DCPD.

### 3.4. SEM images and EDS elemental dot maps

The elemental composition of banana peel (BN), hydroxyapatite (HAp), and the HAp-BN biocomposite was investigated by EDS to confirm the formation and distribution of the brushite phase. As shown in Fig. 7, EDS line-scan analysis of the biocomposite clearly evidences the presence of calcium and phosphorus on the outer surface of the banana peel fibers, confirming the effective in situ deposition of dicalcium phosphate dihydrate (DCPD). The calculated Ca/P molar ratios of approximately 1.0 and 1.67 are in good agreement with the stoichiometric values of brushite and hydroxyapatite, respectively, as reported in the literature [9,37].

<table><caption>**Table 3**
Comparative EDS Analysis of BN, HAp Synthesized, and biocomposite.</caption><thead><tr><th>Element</th><th>Banana Peel (mass%)</th><th>HAp-BN (mass%)</th><th>HAp synthesized (mass%)</th><th>Banana Peel (atom%)</th><th>HAp-BN (atom%)</th><th>HAp synthesized (atom%)</th></tr></thead><tbody><tr><td>C</td><td>56.97 ± 0.18</td><td>6.52 ± 0.11</td><td>6.35 ± 0.11</td><td>64.33 ± 0.20</td><td>10.31 ± 0.18</td><td>11.51 ± 0.20</td></tr><tr><td>N</td><td>0.42 ± 0.09</td><td>—</td><td>—</td><td>0.41 ± 0.09</td><td>—</td><td>—</td></tr><tr><td>O</td><td>40.38 ± 0.35</td><td>60.80 ± 0.48</td><td>42.93 ± 0.50</td><td>34.23 ± 0.30</td><td>72.20 ± 0.57</td><td>58.38 ± 0.69</td></tr><tr><td>Na</td><td>0.68 ± 0.06</td><td>—</td><td>—</td><td>0.40 ± 0.04</td><td>—</td><td>—</td></tr><tr><td>Mg</td><td>0.13 ± 0.06</td><td>—</td><td>—</td><td>0.07 ± 0.03</td><td>—</td><td>—</td></tr><tr><td>Al</td><td>0.28 ± 0.07</td><td>0.20 ± 0.02</td><td>0.14 ± 0.02</td><td>0.14 ± 0.04</td><td>0.14 ± 0.01</td><td>0.11 ± 0.01</td></tr><tr><td>P</td><td>0.15 ± 0.09</td><td>14.06 ± 0.14</td><td>15.98 ± 0.14</td><td>0.07 ± 0.04</td><td>8.62 ± 0.08</td><td>11.22 ± 0.10</td></tr><tr><td>S</td><td>0.08 ± 0.10</td><td>—</td><td>—</td><td>0.04 ± 0.04</td><td>—</td><td>—</td></tr><tr><td>K</td><td>0.91 ± 0.40</td><td>—</td><td>—</td><td>0.31 ± 0.14</td><td>—</td><td>—</td></tr><tr><td>Ca</td><td>—</td><td>18.43 ± 0.19</td><td>34.60 ± 0.26</td><td>—</td><td>8.73 ± 0.09</td><td>18.78 ± 0.14</td></tr><tr><td>Total</td><td>100.00</td><td>100.00</td><td>100.00</td><td>100.00</td><td>100.00</td><td>100.00</td></tr></tbody></table>

Elemental mapping revealed the presence of C and O associated with the lignocellulosic matrix, along with Ca and P originating from the mineral phases. Light elements and trace components from the biomass, such as Na, Mg, K, N, and S, were not clearly detected in the HAp-BN composite, which can be attributed to the intrinsic limitations of EDS for low atomic number elements and their masking by the Ca—P rich brushite layer formed on the biomass surface. In contrast, Al remained detectable due to its higher atomic number and possible contribution from natural biomass traces or minor impurities. It should be noted that EDS provides semi-quantitative information, and the reported elemental ratios reflect general compositional trends rather than phase purity (Table 3) [38].

### 3.5. Elemental analysis

Elemental (CHNS) analysis was performed to determine the mass percentages of carbon, hydrogen, nitrogen, and sulfur in the samples. The elemental composition was obtained by integrating the chromatographic peaks corresponding to each element, allowing the determination of the overall empirical composition of the organic fraction. The measurements were carried out with an absolute accuracy of ±0.4%, and the results are summarized in Table 4.

Each sample is weighed precisely in a tin pan. The standard used for the calibration range is BBOT with the following contents: %C:72.53; %H:6.09; %N:6.51; %S: 7.44%.

### 3.6. HAp-BN biocomposite TG-DT analysis

Thermogravimetric analysis (TGA) is a thermal analysis technique used to measure the variation in mass of a sample subjected to temperature changes over a precise time in a controlled environment related to pressure, atmosphere and temperature. A high-precision balance is used for thermal analysis. A sample of a few milligrams (between 5 and 10 mg) were placed on the measuring rod. The rod is inserted into a furnace where sweeping gas (argon, air, nitrogen, oxygen, etc.) is circulated. The thermal stability was assessed by heating from 30 °C to 800 °C under a continuous nitrogen purge of 10 mL/min at rate 5 °C/min. The resulting TG-DTA curves in Fig. 8 displayed different profiles characterized by different ranges of thermal degradation.

The TG curves for BN and HAp-BN samples shown in Fig. 8 indicate

<table><caption>**Table 4**
Results summary.</caption><thead><tr><th>Ref</th><th>%N</th><th>%C</th><th>%H</th><th>%S</th></tr></thead><tbody><tr><td>HAp syn</td><td>0</td><td>0</td><td>0</td><td>0</td></tr><tr><td>Banan peel</td><td>0*</td><td>44.72</td><td>5.32</td><td>0</td></tr><tr><td>Biocomposite</td><td>0*</td><td>21.76</td><td>3.13</td><td>0</td></tr></tbody></table>

Trace*: detection of the presence of nitrogen but content not quantifiable as less than 0.4%.
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Thermogravimetric (TGA) curves of (a) Banana peel (BN) and (b) HAp-BN biocomposite, recorded under nitrogen atmosphere from 30 to 800 °C at a heating rate of 5 °C.min⁻¹.</figcaption></figure>

that both materials exhibit a multi-step thermal degradation process, typically observed for lignocellulosic biomass by continued weight loss. In the first stage (30–150 °C), the weight loss is attributed to the removal of physically adsorbed water and volatile compounds. For the biomass sample, this accounts for 2.3%, while the HAp-BN biocomposite shows a slightly higher loss of 4.69%, likely due to residual moisture from the synthesis process. The second stage (150–200 °C) corresponds to the release of chemically bound water and the initial degradation of hemi-cellulose. BN exhibits a mass loss of 11.71%, compared to only 1.80% for the biocomposite.

Above 200 °C, the decomposition process proceeds through two major stages of mass loss. The first significant loss, occurring between 200 and 360 °C, corresponds to the thermal breakdown of hemicellulose and cellulose, resulting in weight losses of approximately 43.99% for BN and 30.40% for HAp-BN. The second stage, observed between 360 and 600 °C, is attributed to the thermal degradation of lignin and carbonaceous residues under Nitrogen atmosphere, with corresponding mass losses of 28.05% and 14.47%, respectively, for BN and HAp-BN [39,40]. These results further confirm the stabilizing effect of hydroxyapatite within the biocomposite matrix. This behavior is consistent with the hypothesis that HAp acts as a thermal barrier and promotes char formation by limiting the volatility of degradation products [39].

According to our TG results, there was no significant weight loss of pure HAp when the temperature increased from 30 °C to 800 °C. The weight loss decreased slowly by 0.83% over the test temperature range, and no endo or exothermic peaks up to 800 °C were detected. This means that the HAp powder exhibits excellent thermal stability, making it a promising material for high-temperature applications such as pollutant adsorption and adsorbent regeneration.

<table><caption>**Table. 5**

Specific Surface Areas of HAp<sub>syn</sub>, BN and HAp-BN:</caption><thead><tr><th>Adsorbents</th><th>BET surface area (m².g⁻¹)</th><th>Total pore volume (cm³.g⁻¹)</th><th>Average pore diameter (nm)</th></tr></thead><tbody><tr><td>HAp syn (a)</td><td>4.28</td><td>5.26 * 10⁻²</td><td>49.1</td></tr><tr><td>BN (b)</td><td>0.36</td><td>2.65 * 10⁻³</td><td>29.8</td></tr><tr><td>HAp-BN (c)</td><td>1.37</td><td>6.46 * 10⁻³</td><td>18.9</td></tr></tbody></table>

### 3.7. N₂ adsorption-desorption isotherm analysis

The specific surface area of the adsorbents was determined by using the Brunauer-Emmett-Teller (BET) method, while the pore size distribution was analyzed from the desorption branch of the isotherm. The BET surface areas of the HAp, biomasse and HAp-BN biocomposite are summarized in Table.5. The nitrogen adsorption-desorption isotherms, interpreted in line with IUPAC guidelines [41], all simples correspond to type IV profiles characterized by clear hysteresis loops, which are typical of materials with mesoporous and macroporous features Fig. 9.

A notable difference in BET surface area was observed among the samples. The untreated Hydroxyapatite Fig. 9(a) reached 4.28 m².g⁻¹, while Banana peel Fig. 9(b) had the lowest value at (0.36 m².g⁻¹). The biocomposite HAp-BN Fig. 9(c) exhibited an intermediate surface area at (1.37 m².g⁻¹). This suggests that the combination of the two phases leads to some degree of pore blockage or surface coverage of the organic component, as the mineral phase (brushite) was deposited onto the Banana peel matrix, resulting in partial obstruction of its porous structure. This phenomenon has also been reported in other recent studies on hybrid biomaterials [42]. For the biocomposite, the BET analysis indicates multilayer adsorption, as expected for type IV isotherms, which is consistent with the presence of mesoporous structures [43].

Further analysis of pore size and total pore volume confirmed that mesopores (2–50 nm) and macropores (≥50 nm) is suggested indirectly by the increase in adsorption at high relative pressure (P/P₀ close to 1) in the isotherms. Total pore volumes ranged from 2.65 × 10⁻³ to 0.0526 cm³.g⁻¹ depending on the analysis method BJH and NLDFT. The biocomposite's pore structure appears more uniform than that of the raw Banana peel, but less open than hydroxyapatite, which could affect the diffusion of molecules through the material. These observations are consistent with recent advances in biomaterials research, where adjusting porosity and surface area through the creation of organic-inorganic hybrids is seen as a promising approach for improving performance in biomedical and environmental contexts [44].

### 3.8. Quantum chemical calculation

Density Functional Theory (DFT) is a quantum chemical method widely used to study the electronic properties of molecules. It is applied to predict molecular stability, chemical reactivity and interactions with the surrounding environment at the atomic scale.

Electronic properties of the cellulose molecule were examined in this part of the work, including HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) plots, as well as reactivity predictions through Fukui function analysis, to explore the interactions of the molecular system with its environment. Cellulose was selected as a representative polysaccharide due to its abundance in Banana peel and its well-defined chemical structure, which allows reliable calculation of Fukui functions. The identified electron-donating and electron-accepting sites associated with hydroxyl groups are also common to other biomass constituents such as hemicellulose and pectin. Via the Gaussian 16 program package [45], geometry optimization of the cellulose molecule was performed using Density Functional Theory (DFT) [46], at the B3LYP level with the 6–311 + G(d,p) basis set [34,47].

The optimized structure exhibits C₁ symmetry, indicating the absence of molecular symmetry constraints. A calculation of harmonic
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Pore size distribution curves derived from nitrogen adsorption-desorption isotherms of (a) hydroxyapatite (HAp), (b) Banana peel biomass (BN), and (c) HAp-BN biocomposite, measured at 77 K and analyzed using the BET method and the desorption branch according to IUPAC classification.</figcaption></figure>

vibrational frequencies was performed to confirm that the structure corresponds to a veritable minimum on the potential energy surface, as shown by the absence of imaginary frequencies. Fukui functions were calculated for the oxygen atoms located at both sides of the cellulose molecule to identify potential interaction sites, based on natural population analysis performed at the same level of theory. The following equations describe the procedure used to determine the Fukui functions [48], corresponding to atom k in the molecule:

$$f_k^+ = [q_k(N+1) - q_k(N)] \text{ for nucleophilic attack,}$$

$f_k^- = [q_k(N) - q_k(N-1)]$ for electrophilic attack, where N is the number of electrons of the aim molecule and $q_k(N+1)$, $q_k(N-1)$ and $q_k(N)$: are the atomic charges for the anion and cation molecules, respectively. Sites in molecules correspond to large values of $f^+(f^-)$ are more favorable to nucleophilic (electrophilic) attack.

The optimized geometry of the cellulose molecule is shown in Fig. 10, along with the corresponding atom labels. As illustrated, several hydroxyl groups participate in intramolecular hydrogen bonding, which may reduce their ability to interact with the external environment, more
<figure><img src="image_13.png" /><figcaption>Fig. 9. (continued).</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 10. Optimized geometry of the cellulose molecule along with the corresponding atom labels.</figcaption></figure>

specifically with the hydroxyapatite (HAp).

To further investigate the interaction potential of these groups, electron density distribution plots of the frontier molecular orbitals HOMO and LUMO were generated and are presented in Fig. 11.

As seen in Fig. 11, it can be observed that the HOMO is primarily localized on the upper right side of the molecule, including the oxygen atom at the molecular center (O23) and the carbonyl oxygen (O24) of the left-side ring.

Conversely, the LUMO is mostly located on the underside of the molecule, particularly around the two hydroxyl groups involved in key intramolecular hydrogen bonding in this region. Given that HOMO represents electron-dense regions capable of donating electrons (nucleophilic sites) and LUMO represents regions capable of accepting electrophilic sites), the results indicate that the upper part of the cellulose molecule corresponding mainly to the oxygens of the hydroxyl groups (O3) and (O40), the central oxygen (O23) and the carbonyl oxygen (O24) act as an electron-donating region. Conversely, the lower part of the molecule, corresponding to the hydroxyl groups comprising oxygens (O26) and (O34), as indicated by the LUMO distribution in Fig. 11, seems to be more inclined to accept electrons, identifying it as an electron-accepting region.

As observed in Table 6, significant values of the electrophilic Fukui descriptor ($f^+$) are found at the O26, O27, and O34 sites, located on the lower side of the cellulose molecule. In contrast, only the O3 site on the

<figure><img src="image_15.png" /><figcaption>Fig. 11. Plots of the cellulose molecule's HOMO and LUMO density distributions calculated using Density Functional Theory (DFT, B3LYP/6-311++G (d,p)).</figcaption></figure>

<table><caption>Table. 6
Fukui functions ($f^+$, $f^-$) for cellulose, using NPA atomic charges.</caption><thead><tr><th></th><th>O3</th><th>O23</th><th>O24</th><th>O25</th><th>O26</th><th>O27</th><th>O34</th><th>O37</th><th>O40</th></tr></thead><tbody><tr><th>f<sup>+</sup></th><td>0.37</td><td>0.31</td><td>0.33</td><td>0.32</td><td>0.39</td><td>0.37</td><td>0.37</td><td>0.36</td><td>0.36</td></tr><tr><th>f<sup>-</sup></th><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td></tr></tbody></table>

upper side of the molecule is predicted to be electrophilic. Additionally, no significant nucleophilic sites were identified based on the nucleophilic descriptor values ($f^-$). These results suggest that the region containing O26, O27, and O34 can act as an electron-accepting
<figure><img src="image_16.png" /><figcaption>**Fig. 12.** Proposition of the biocomposite formation mechanism.</figcaption></figure>

(electrophilic) area and a preferred site for nucleophilic attack, in contrast, only one electrophilic site (O3) is found on the opposite side, which could be susceptible to electrophilic attack. These observations are consistent with the previously discussed HOMO and LUMO electron density distributions and indicate that the upper side of the cellulose molecule is the preferred interaction region for Ca²⁺ ions of hydroxyapatite (HAp) during biocomposite formation.

### 3.9. Proposition of the biocomposite formation mechanism

The formation mechanism of the HAp-BN biocomposite was proposed using a combination of software, including BioRender, PowerPoint, and Gaussian, to visualize the molecular interactions involved. These simulations aim to identify the nature of the interactions involved between the functional groups such as carboxylic groups, hydroxyl

groups, and ether oxygen atoms present in the biomasse, and the calcium ions exposed on the surface of DCPD crystal.

Fig. 12 illustrates the dipolar ionic interactions between calcium ions and carboxylate groups on the BN surface trigger nucleation [49]. The negative charges of the carboxylate groups are readily associated with the positive charges of the calcium ions. Additionally, coordination and complexation occur between calcium ions and other oxygen-containing functional groups on BN, notably hydroxyl groups (–OH), carbonyl groups (C=O), and ether oxygen atoms (–C–O–C–). These groups act as electron donors, forming coordination bonds with Ca²⁺ ions on the brushite surface, which further stabilizes the biocomposite structure. Another important interaction is hydrogen bonding between brushite and hydroxyl groups of the biomaterial, occurring both intra- and intermolecularly [50].

### 3.10. HAp-BN biocomposite adsorption performance

To evaluate the adsorption performance of the biocomposite, an adsorption study of Direct Yellow 177 GLE was used as the model dye. This anionic dye, identified by the Color Index (CI) number 26620, has molecular formula C₃₈H₂₅N₆Na₃O₁₁S₃. The textile dye was conducted using three types of adsorbent materials: Banana peel (BN), hydroxyapatite (HAp), and their biocomposite (HAp-BN), with adsorbent dosage ranging from 0.01 g to 0.2 g. The experiments were conducted at ambient temperature (25 °C), maintaining a constant initial dye concentration of 25 mg.L⁻¹, a contact time of 2 h, and an initial pH between 7 and 7.5. The Dye uptake measurements were performed using a UV–Vis spectrophotometer (model 1601 PC, SHIMADZU).

Fig. 13 illustrates the evolution of the adsorption efficiency as a function of the adsorbent dosage for each of the materials studied. The HAp-BN biocomposite exhibits a superior adsorption capacity compared to the individual materials (BN and HAp).

According to the results obtained, the yield of elimination of the DY177 dye in aqueous solution by the HAp-BN biocomposite (96%) is very high, compared with that of hydroxyapatite and Banana peel, which are 67% and 36%, respectively. This could be explained by the high availability of active adsorption sites in the biocomposite.

## 4. Conclusion

The present study successfully reports the synthesis of a novel HAp-

<figure><img src="image_17.png" /><figcaption>**Fig. 13.** Impact of adsorbent dosage on the sorption efficiency of the dye Direct Yellow 177 GLE using HAp, BN and HAp-BN, at 25 °C, initial concentration 25 mg. L⁻¹, contact time 2 h, pHᵢ = 7.</figcaption></figure>
BN biocomposite based on hydroxyapatite and Banana peel biomass. The biocomposite was thoroughly characterized using complementary physicochemical, structural, and spectroscopic techniques, allowing a clear understanding of its composition and structure. The analysis confirmed the in-situ formation of brushite (DCPD) on the lignocellulosic surface of the biomass, leading to a hybrid material with optimized textural, morphological, and surface functional properties. The coexistence of chemically active groups such as -OH, -COOH, and $HPO_4^{2-}$ significantly contributes to the enhanced surface reactivity of the biocomposite and improve its adsorption capability.

Density Functional Theory (DFT) calculations provided insights into the interactions governing biocomposite formation. The analysis of HOMO-LUMO orbitals and Fukui reactivity indices identified specific cellulose sites favorable for electron donation and acceptance, supporting the proposed formation mechanism involving dipolar ionic and coordination bonding, and hydrogen bonding between biomass functional groups and $Ca^{2+}$ ions on the DCPD surface. These results strengthen the mechanistic interpretation of the experimental observations and highlight the relevance of combining theoretical and experimental approaches.

Under similar experimental conditions, the HAp-BN biocomposite was significantly more effective (97%) at removing the dye compared to synthesizing hydroxyapatite (67%) and biomass (36%) alone. This improvement is attributed to the synergistic effect between the mineral phase and the biomass, which increases the density and accessibility of active sites and enhances ion-exchange and surface interaction mechanisms.

Beyond the reported results, this study demonstrates the scientific relevance of inducing controlled phase transformation from HAp to brushite in the presence of biomass as a strategy to tailor surface chemistry and functionality. The findings provide a solid foundation for the rational design of eco-friendly and cost-effective biocomposite materials. Future work will focus on detailed adsorption kinetics, isotherm modeling, stability and regeneration studies, as well as validation using real industrial wastewater, to fully assess the practical applicability of the HAp-BN biocomposite for water treatment applications.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

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