Biochar–silver nanocomposites derived from Borassus flabellifer bark for rapid catalytic reduction of organic pollutants in wastewater

Abstract

Rapid industrial growth has met human needs but has also led to the contamination of water bodies with persistent and toxic pollutants such as dyes, pharmaceutical waste and heavy metals. Although noble metal-based nanocatalysts have been widely used for pollutant removal, their low efficiency and high cost have prompted the search for affordable and efficient alternatives for wastewater treatment. Biochar-based metal composites have gained significant attention owing to their low cost and effective pollutant removal. In this work, biochar–silver composites (Ag@PBC) were synthesized from Borassus flabellifer bark biomass through pyrolysis at varying temperatures (300–600 °C), using silver nitrate as the source of silver (Ag⁺) ions. The properties and structures of the materials were extensively characterized using UV-DRS, FTIR, PXRD, XPS, HR-SEM, HR-TEM, and BET. TEM analysis confirmed the deposition of silver nanoparticles (5–15 nm) on the biochar surface. Moreover, the catalytic performance of the Ag@PBC composites was tested for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP), as well as the reduction of dyes methyl orange (MO) and Congo red (CR) in aqueous media, with sodium borohydride (NaBH₄) serving as the reducing agent. Among the four composites, Ag@PBC 500 exhibited the highest efficiency, achieving the rapid reduction of 4-NP and dyes (MO and CR) within 3 min and 60 s, respectively. The catalytic reduction proceeded efficiently in a mild alkaline medium, with an optimum pH range of 8–11. These results suggest that Ag@PBC composites are promising catalysts for removing nitroaromatic compounds and dyes from industrial wastewater. This study provides an eco-friendly and cost-effective alternative for environmental remediation.

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1. Introduction

Rapid industrial growth has met human needs. On the other hand, industrial effluents exposed to water bodies can cause water to be contaminated, thus posing serious environmental issues.¹ Contamination of water by toxic compounds like 4-nitrophenol (4-NP), methyl orange (MO), and Congo red (CR) discharged from the leather industries, pharmaceutical industries, dye industries, and textile industries causes severe problems because of their persistent and bio-accumulative nature.^2–5 These compounds are well reported to cause cytotoxicity, carcinogenic, mutagenic and teratogenic effects in humans and animals.⁶ Additionally, 4-NP and CR cause skin and eye irritation, nausea and headaches^7,8 and MO causes intestinal cancer.⁹ In response to these concerns, several remediation strategies have been investigated. Among these, photocatalysis and adsorption are the two most employed conventional methods to remove pollutants in wastewater treatment.^10,11 However, these methods encounter limitations like high cost, low efficiency, secondary contamination, low rate of removal and recovery in their practical application for wastewater treatment.¹² In contrast, NaBH₄-assisted chemical reduction offers a significantly more effective and eco-friendly method for the removal of pollutants in wastewater treatment.^13,14

Noble metal nanoparticles (NPs) are significant catalysts for pollutant remediation, owning a high surface area to volume ratio and electron transfer. Because of these properties, these nanoparticles show high reduction efficiency in the reduction of 4-NP, MO and CR.¹⁵ For example, Islam et al. (2018) synthesized different nanoparticles (Au, Ag, Pt, and PdNPs) in one-step using sodium rhodizonate as both a reducing and stabilizing agent. The synthesized Au and Ag nanoparticles showed effective catalytic activity in reducing 4-nitrophenol, and the Pt nanoparticles showed high reducing efficiency in methyl orange reduction.¹⁶ Further research by Ecer et al., (2024) synthesized a Bi (0)-doped hydroxyapatite/reduced graphene oxide (Bi/HAp–rGO) catalyst for the rapid reduction of methyl orange (MO) in the presence of NaBH₄. The catalyst showed excellent performance with 99.6% decolonization efficiency under optimized conditions within just 2.91 minutes.¹⁷ Similarly, Alene et al. (2025) synthesized boron-doped cobalt oxide (B@Co₃O₄) nanoparticles via a co-precipitation method for efficient Congo red (CR) dye degradation. The catalyst exhibited a high surface area (548.1 m² g⁻¹), reduced band gap (1.94 eV), and improved thermal stability. In the presence of NaBH₄, B@Co₃O₄ achieved 99.91% CR removal within 6 minutes, following pseudo-first-order kinetics. The catalyst maintained >98% efficiency over ten reuse cycles, confirming its durability.¹⁸ Despite the efficiency of noble metal nanoparticles, they often undergo aggregation and oxidation, and this reduces their catalytic efficiency and stability.¹⁹

Recently, biochar-based metal composites have received more attention as heterogeneous catalysts in NaBH₄-assisted chemical reduction because of their efficient electron transfer and robust adsorption capacity.^20,21 Biochar is a carbonaceous material that is produced from the decomposition of biomass in an oxygen-free atmosphere.^22,23 Due to its porous structure, surface functionality and enhanced adsorption capacity, it is used as an adsorbent and heterogeneous catalyst in wastewater treatment.^24,25 However, certain biochars have the inherent limitations of low surface area, low surface functionality and low adsorption capacity, which may reduce their efficiency in pollutant removal.^26,27 Integrating the nanoparticles into biochar enhances the stability and the overall reduction efficiency of the catalyst by generating an active site for redox reaction and a large surface area, which promotes the breakdown of pollutants such as nitroaromatics and dyes.^28,29 Moreover, noble metal nanoparticles (Ti, Pt, Pd, Co, Ru, Os, Au and Ag), are widely studied but except for relatively inexpensive silver.^30,31 As a result, silver nanoparticles are commonly used as heterogeneous catalysts to remove pollutants. Jacob et al., (2024) prepared a calcite-rich biochar composite derived from the bark of Tamaridus indica, which successfully supported silver nanoparticles and enabled 4-NP reduction in just 24 minutes at room temperature.³²

Among the various biomass sources, Borassus flabellifer bark was selected as the biomass precursor owing to its abundant availability, renewable nature and lignocellulosic richness (cellulose, hemicellulose, and lignin), making it a promising candidate for sustainable carbon material production. While the exact proportions of these components for the bark specifically are less documented, studies on other lignocellulosic palm fibers show a general range of 45–70% cellulose, 14–32% hemicellulose, and 5–30% lignin.³³ During carbonization, the bark cellulose and hemicellulose create a porous carbon matrix, while lignin contributes to the formation of aromatic carbon networks. The resulting biochar features a high surface area and is rich in oxygen-containing functional groups, such as hydroxyl (–OH) and carboxyl (–COOH).^34,35 These functional groups are excellent anchoring (deposition) sites for silver nanoparticles, promoting uniform dispersion and stabilized electron transfer during catalytic reactions, such as the reduction of 4-nitrophenol and dyes. Additionally, the naturally present mineral ash (e.g., K, Ca, Mg) acts as an inherent activation agent and enhances the surface activity of the biochar. Compared to commonly used biomasses such as coconut shell, rice husk, or sawdust, B. flabellifer bark yields higher carbon content and improved pore structure, providing more active sites for metal nanoparticle anchoring. Therefore, its utilization represents a novel and locally available carbon source that supports sustainable material design and aligns with sustainable development goal 12 (responsible consumption and production).

In this work, we prepared four silver–biochar composites from Borassus flabellifer biomass by varying the pyrolysis temperatures (300–600 °C) using silver nitrate as the Ag⁺ source by a chemical reduction method. The composites were well characterized using state-of-the-art techniques. The catalytic propensity of silver–biochar composites was studied by the reduction of 4-nitrophenol (4-NP), methyl orange (MO) and Congo red (CR) as model catalytic reactions. The study highlights Ag@PBC as a promising catalyst for the removal of both nitroaromatic compounds and dyes, due to the synergetic effect of biochar and silver nanoparticles.

2. Materials and methods

2.1. Materials

All chemicals utilized in this study were of ACS grade and were used without any further purification. Silver nitrate (AgNO₃), 4-nitrophenol (4-NP), sodium borohydride (NaBH₄), methyl orange (MO), Congo red (CR) and phosphoric acid (H₃PO₄) were procured from SRL chemicals. All experimental procedures utilized deionized water.

2.2. Biochar preparation

Borassus flabellifer bark was collected from the rural area (Pannaivillai bungalow, Thoothukudi). Deionized water was used to wash the bark, and it was later dried under sunlight, and ground into a fine powder. 10 g of the powder was soaked in 1 N phosphoric acid for 24 h. The mixture was filtered and rinsed with distilled water until the phosphoric acid was left out. Then the resultant residue was dried in a hot air oven at 80 °C for 24 h. Subsequently, 5 g of dried material was carbonized in a muffle furnace at different temperatures ranging from 300 to 600 °C for 4 hours to obtain four different biochar samples. After carbonization, the furnace was allowed to cool naturally to room temperature. The resulting biochar samples were labelled as PBC 300, PBC 400, PBC 500 and PBC 600, where the suffix indicates the temperature used to produce each sample. Finally, the samples were ground well using a mortar and pestle to obtain a fine powder.

2.3. Preparation of the composites

1 g of biochar powder was added to a solution containing 1 mM of silver nitrate. Then, 2 mM of sodium borohydride solution was added rapidly under continuous stirring using a magnetic stirrer under cold conditions. The mixture was stirred for 20 minutes to facilitate the uniform reduction of silver ions and then allowed to settle. The obtained mixture was decanted and then dried at 80 °C for 4 h to obtain a composite. This process facilitates the reduction of Ag⁺ ions to metallic silver (Ag⁰) by sodium borohydride, resulting in the deposition of silver nanoparticles onto the biochar surface. The corresponding chemical reaction is given below:

2AgNO₃ + 2NaBH₄ → 2Ag + B₂H₆ + 2NaNO₃ + H₂

2.4. Characterization

Thermogravimetric analysis (TGA) was employed to investigate the thermal degradation behavior of the precursor and determine the optimal temperature for the biochar preparation using a NETZSCH thermogravimetric analyzer from room temperature up to 1300 °C. Powder X-ray diffraction (P-XRD) was performed to analyze the phase and crystal structure of the biochar and composite, using PANalytical Aeries in continuous scanning mode. The samples were analyzed over an angular range of 2θ = 10°–80°. Further UV-DRS was performed at ambient temperature using the UV-2400 PC Series, and the spectral range covered 200–800 nm. Fourier transform infrared (FTIR) spectroscopy was conducted to find the surface functionalities of the prepared composites with a Shimadzu Irtracer 100 over the spectral range of 400–4000 cm⁻¹. The surface composition and oxidation state of the composites were analyzed using X-ray photoelectron spectroscopy (XPS). The morphological characteristics of the prepared biochar and silver–biochar composites were examined using scanning electron microscopy (SEM), and the elemental compositions were analyzed via energy-dispersive X-ray spectroscopy (EDS) using the Apreo S LoVac instrument and high-resolution transmission electron microscopy (HR-TEM) using JOEL. To study the surface area, total pore volume, and pore diameter of the composite, BET analysis was performed using a BET analyzer.

2.5. Catalytic reduction of 4-nitrophenol and azo dyes

To evaluate the catalytic efficiency of the prepared composites, the reduction of 4-nitrophenol and the dyes (methyl orange and Congo red) was carried out. In a typical procedure, 20 mL of (0.2 mM) 4-nitrophenol was treated with sodium borohydride (0.2, 1.0, 2.0, 5.0, 10.0, 15.0, 20.0 mM) and exposed to constant stirring at room temperature. After the addition of sodium borohydride, the solution color changes from pale yellow to dark yellow, due to the formation of a 4-nitrophenolate ion. 1 mg of composite was added to the solution and exposed to constant stirring. The reaction process was monitored using the UV-vis adsorption spectroscopy at constant intervals. The conversion of 4-nitrophenol to 4-aminophenol can be observed by a visible color change from yellow to colorless, indicating the completion of the reaction.

For the reduction of azo dyes (MO and CR) at varying concentrations (25 ppm, 50 ppm, and 100 ppm), 20 mL of freshly prepared NaBH₄ (0.2 mM) was added to 20 mL of the respective dye solution. Afterwards, 1 mg of the composite was added and the reaction mixture was stirred continuously. The catalytic reaction was monitored by UV-vis spectroscopy at fixed time intervals to observe the decrease in absorbance corresponding to the dye degradation.

A mixed pollutant solution was prepared with 0.2 mM of 4-NP, 100 ppm of MO, and CR in actual groundwater to emulate aquatic environmental conditions. From this solution, 20 mL was treated with 20 mL of freshly prepared NaBH₄ (0.20 mM), followed by the addition of 1 mg of composites. The reaction mixture was stirred continuously with a magnetic stirrer and the reduction process was monitored using UV-vis spectroscopy. This study helps to evaluate the catalyst performance in real and complex conditions.

3. Result and discussion

The preparation of the silver–biochar composite involves three steps: (1) biochar preparation (PBC) – powdered Borassus flabellifer bark was pretreated with phosphoric acid (H₃PO₄) to improve carbon retention and adsorption capacity of the biochar. As shown in Fig. 1, the TGA graph reveals that major weight loss occurs between 300 and 600 °C corresponding to the active decomposition of lignocellulosic biomass, which states that the biochar prepared at this temperature range has high carbon content and defined structure. Based on the TGA results, the pretreated material has been carbonized at a temperature range of 300 to 600 °C. The biochar yield decreases from 56% to 19% with an increase in temperature from 300 to 600 °C. This suggests that biochar preparation is influenced by the pyrolysis temperature, with a higher temperature leading to greater thermal degradation and lower yield. (2) Adsorption of Ag⁺ ions – the biochar is added to the silver nitrate solution, allowing Ag⁺ ions to adsorb onto the biochar (PBC) surface and (3) reduction of Ag⁺ – by using sodium borohydride, the Ag⁺ ions undergo chemical reduction (Ag⁺ to Ag⁰), yielding the catalyst Ag@PBC.³² The Ag@PBC composites are prepared via a simplified and eco-friendly route that involves phosphoric acid treatment, which enhances the biochar's porosity and surface functionality, thereby promoting efficient Ag⁺ deposition. Subsequent NaBH₄-assisted reduction ensures uniform dispersion of Ag nanoparticles, making the process low-cost and scalable. Fig. 2 illustrates the preparation step of PBC and Ag@PBC. The characterization of PBC and Ag@PBC is discussed below.

Fig. 1 TGA curve of Borassus flabellifer bark biomass.

Fig. 2 Schematic representation of the preparation of Ag@PBC composites.

3.1. Powder X-ray diffraction

Powder X-ray diffraction (PXRD) is a crucial tool for materials like biochar to determine the phase within the materials. Fig. 3 represents the XRD pattern of biochar (PBC) and silver–biochar composites (Ag@PBC) synthesized at different pyrolysis temperatures of 300–600 °C. All biochar (PBC) XRD patterns exhibit broad diffraction at 22°, indicating the amorphous phase corresponding to the (002) planes of the turbostratic carbon structure.³⁶ In contrast, the XRD pattern of the silver–biochar composites shows characteristic peaks around 37°, 44°, 64° and 77°, which were indexed as the (111), (200), (220), and (311) planes of face-centred cubic silver, respectively (JCPDS card no. 04-0783).³⁷ These results indicate that the biochar exhibits an amorphous carbon structure and the appearance of silver peaks confirms the successful reduction of silver ions on the biochar surface.

Fig. 3 XRD patterns of PBC and Ag@PBC composites prepared at different pyrolysis temperatures (300–600 °C).

3.2. UV-Visible spectroscopy

UV-Visible spectroscopy is considered as a basic and efficient tool for confirming the successful reduction of metal ions to nanoparticles. As shown in Fig. 4, the spectra of the Ag@PBC composites, a broad peak beyond 390 nm without an absorption maximum suggests the large distribution of silver nanoparticles.³⁸ In addition to that, two peaks were observed at 293 nm and 343 nm because of the π–π* and n–π* transitions originating from C=C and C=O groups, respectively. However, with the increase in pyrolysis temperature from 300 to 600 °C, the peaks corresponding to C=C and C=O were diminished, indicating the destruction of the structure of the biochar.

Fig. 4 UV-Vis spectra of Ag@PBC composites: (a) Ag@PBC 300, (b) Ag@PBC 400, (c) Ag@PBC 500 and (d) Ag@PBC 600.

3.3. Fourier-transform infrared (FT-IR) spectroscopy

FT-IR spectroscopy was used to determine the surface functionality of biochar and its composites (Ag@PBC). The FT-IR spectra of both biochar (PBC) and Ag@PBC are shown in Fig. 5. In biochar and Ag@PBC, the broad band obtained at ∼3400 cm⁻¹ corresponds to O–H stretching vibrations of alcoholic and phenolic groups. In addition to that, the band obtained at ∼1602 cm⁻¹ is attributed to the stretching vibration of C=O groups (carbonyl or carboxylic functionalities). The band observed around ∼1595 cm⁻¹ corresponds to the aromatic C=C vibrations of biochar. The presence of a band at ∼1082 cm⁻¹ can be attributed to the aromatic C–O stretching vibration. The vibrational bands below 800 cm⁻¹ were mainly due to the C–H bending vibration of aromatic and heteroaromatic compounds present in the biochar.³⁹ From the above findings, the presence of hydroxyl functional groups in biochar and its composites is confirmed and characterized.

Fig. 5 FTIR spectra of PBC and Ag@PBC composites prepared at different pyrolysis temperatures (300–600 °C).

3.4. X-ray photoelectron spectroscopy

XPS analysis was employed to determine the composition and valence state of the prepared Ag@PBC. XPS spectra for all Ag@PBC are given in Fig. 6 and Fig. S1–S3. The survey spectra for the composites indicate the presence of carbon (C 1s), oxygen (O 1s) and trace levels of calcium and silver (Ag 3d), as shown in Fig. 6a and Fig. S1a–S3a. Among these composites, the carbon atomic percentage decreases from 76% (Ag@PBC 600) to 61.7% (Ag@PBC 400). The oxygen percentage increases from 23% to 37%, which indicates that higher pyrolysis temperature reduces the oxygen functional groups. Notably, the presence of oxygen functional groups on the biochar surface can promote the deposition of metal ions through ion exchange and electrostatic interaction.⁴⁰Fig. 6b and Fig. S1b–S3b show that the C 1s XPS spectra have been deconvoluted into four distinct peaks. The peak at 284.6 eV corresponds to the sp² hybridization of graphene (C–C/C=C), while the peak at 285.5 eV is associated with the C–O bond. The peaks observed at 286.5 and 288.5 eV are attributed to C=O and O–C=O bonds, respectively. From Fig. 6(c) and Fig. S1(c)–S3(c), it is revealed that the O 1s XPS spectral peaks at 531.5, 532.3 and 533.3 eV are attributed to C–O, C=O and O–C=O, respectively. Fig. 6d and Fig. S1d–S3d show prominent peaks at 374.8 eV (Ag 3d_3/2) and 367.4 eV (Ag 3d_5/2) arising due to the presence of silver at a reduced state, which confirms the deposition of silver nanoparticles onto the biochar surface.^41,42Table 1 shows the XPS peak positions and assignments of Ag@PBC composites at different pyrolysis temperatures.

Fig. 6 XPS spectra of Ag@PBC-500 composite (a) survey spectrum, (b) C 1s, (c) O 1s and (d) Ag 3d.

Table 1 XPS peak positions and assignments of the Ag@PBC composites at different pyrolysis temperatures

Composites	Peaks	Position (eV)	Assignment
Ag@PBC 300	C 1s	284.6	C–C/C=C
		285.5	C–O
		286.5	C=O
		288.5	O–C=O
	O 1s	531.5	C–O
		532.3	C=O
		533.3	O–C=O
	Ag 3d	374.8	Ag 3d3/2
		367.4	Ag 3d5/2
Ag@PBC 400	C 1s	284.6	C–C/C=C
		285.5	C–O
		286.5	C=O
		288.5	O–C=O
	O 1s	531.5	C–O
		532.3	C=O
		533.3	O–C=O
	Ag 3d	374.8	Ag 3d3/2
		367.4	Ag 3d5/2
Ag@PBC 500	C 1s	284.6	C–C/C=C
		285.5	C–O
		286.5	C=O
		288.5	O–C=O
	O 1s	531.5	C–O
		532.3	C=O
		533.3	O–C=O
	Ag 3d	374.8	Ag 3d3/2
		367.4	Ag 3d5/2
Ag@PBC 600	C 1s	284.6	C–C/C=C
		285.5	C–O
		286.5	C=O
		288.5	O–C=O
	O 1s	531.5	C–O
		532.3	C=O
		533.3	O–C=O
	Ag 3d	374.8	Ag 3d3/2
		367.4	Ag 3d5/2

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3.5. Scanning electron microscopy (SEM)

One of the essential parameters for characterizing a biochar sample is analyzing its surface morphology, as it provides critical insights into the surface characteristics. Fig. 7 represents the surface morphology of the prepared biochar (PBC), and silver–biochar composite (Ag@PBC) prepared at different pyrolysis temperatures (300–600 °C). Upon first observation, the biochar surface appeared highly porous and irregular.

Fig. 7 SEM micrographs of (a)–(d) PBC and (e)–(h) Ag@PBC composites.

The biochar [Fig. 7(a)–(d)] exhibits a rough, porous, and layered-like surface morphology, characteristic of biomass-derived carbon materials, indicating partial biomass degradation during pyrolysis.⁴³ This porous morphology is advantageous for various applications, especially catalysis and adsorption, due to enhanced surface area and potential active sites. In contrast, SEM images of the Ag@PBC composites [Fig. 7(e)–(h)] reveal notable changes in surface with visible spherical or agglomerated particles embedded within the carbon matrix. These particles are attributed to silver nanoparticles (Ag NPs) successfully deposited on the biochar surface. However, the addition of silver does not affect the surface of the biochar. The EDS spectra further validate the elemental composition of both materials, as shown in Fig. S4(a) and (b). The spectrum of biochar (Fig. S4a) predominantly shows peaks for carbon (C) and oxygen (O), confirming the carbonaceous nature of the material. After silver addition, the EDS spectrum (Fig. S4b) shows additional, prominent peaks corresponding to Ag, thereby confirming the presence of silver in the composite.

3.6. Transmission electron microscopy (TEM)

Transmission electron microscope (TEM) analysis was employed to understand the morphological patterns and structural features of the prepared composites; the results are shown in Fig. 8. Fig. 8(a) and Fig. S5(a)–S7(a) reveal a large amount of deposition of the silver nanoparticles (dark circle) without any sign of aggregation on the biochar. According to Fig. 8(b), (c) and Fig. S5(b), (c)–S7(b), (c), the as-synthesized silver nanoparticles are spherical and uniformly distributed with sizes typically ranging from 5–15 nm. The darker contrast of these particles indicates their crystalline nature compared to the amorphous nature of biochar.⁴⁴ Additionally, the selected area electron diffraction (SAED) patterns in Fig. 8(d) and Fig. S5(d)–S7(d) exhibit rings composed of bright spots, suggesting the crystalline nature of the silver nanoparticles, conveying that AgNPs are in the nm range and distributed randomly on the biochar surface. Furthermore, it shows that the crystal lattice planes can be indexed as (111), (200), (220) and (311), matching the XRD results.^45,46 The presence of a diffuse background further reveals the amorphous nature of the biochar. Thus, the TEM analysis reveals the successful deposition of silver nanoparticles onto the surface of PBC. The observed spherical morphology of the silver particles provides a higher active surface area, which could make it an excellent catalyst.

Fig. 8 TEM images of the Ag@PBC 500 composite at different magnifications (a)–(c) and the SAED pattern (d).

3.7. Brunauer–Emmett–Teller (BET)

The surface area and porous structure of the prepared biochar (PBC) and its silver-loaded composites (Ag@PBC) were investigated using nitrogen adsorption–desorption isotherms. As shown in Fig. S8, all samples exhibit typical type IV isotherms with H4-type hysteresis loops, characteristic of mesoporous materials with narrow slit-shaped pores. The presence of narrow hysteresis loops further suggests the formation of open-structured mesopores.^47,48 The BET results show that the surface area of the biochar increased from 142.609 to 185.495 m² g⁻¹ as the pyrolysis temperature was raised from 300 to 600 °C. In addition, the incorporation of silver nanoparticles significantly enhanced the surface area, with Ag@PBC 300 and Ag@PBC 600 exhibiting surface areas of 180.308 and 201.328 m² g⁻¹, respectively. The total pore volumes of PBC 300 and PBC 600 were found to be 0.309 and 0.372 cm³ g⁻¹, while Ag@PBC 300 and Ag@PBC 600 showed values of 0.429 and 0.629 cm³ g⁻¹, respectively. Cao et al. prepared biochar derived from pomegranate peel at 300 and 600 °C, which showed relatively lower surface areas (41.28 and 195.32 m² g⁻¹, respectively).⁴⁹ Similarly, Tian et al. reported Ag composites supported on advanced carbon materials such as Ag/RGO (182.2 m² g⁻¹) and Ag/N–RGO (165.8 m² g⁻¹), show lower surface area compared to our Ag@PBC composites.⁵⁰ Furthermore, Eltaweil et al. synthesized Ag@biochar using Chenopodium ambrosioides, where the incorporation of Ag led to a decrease in surface area.³⁸ Our composites show an increase in surface area, confirming homogeneous Ag dispersion and preservation of the porous framework. Overall, the BET analysis reveals that increasing the pyrolysis temperature and incorporating Ag nanoparticles contribute to a more developed porous structure with higher surface area. These features are expected to enhance adsorption capacity and promote efficient interaction with target pollutants during catalytic applications.

4. Catalytic reduction

4.1. 4-Nitrophenol reduction

Due to its straightforward reaction, the reduction of 4-NP serves as a model reaction to investigate the catalytic propensity of various biochar–metal composites. It is readily observable via UV-vis absorbance spectroscopy under room conditions. In addition to that, the reduction reaction can be observed with the naked eye because of a visible color change from yellow to colorless.⁵¹ However, the reduction is thermodynamically favorable and it is hindered by its kinetic barrier necessitating the use of a catalyst. The prepared composites act as a catalyst to reduce the kinetic barrier of the reduction reaction.⁵²

Upon addition of sodium borohydride to an aqueous 4-NP solution, the pale-yellow color was darkened, indicating the formation of 4-nitrophenolate ions because of the deprotonation of 4-NP. As shown in Fig. 9, a bathochromic shift in the absorption peak from 353 nm (4-NP) to 400 nm (4-nitrophenolate ion) further evidenced the conversion.^53,54 However, after the addition of NaBH₄ there was no significant change in absorption maximum, indicating zero conversion in the reaction medium. 1 mg of catalyst (Ag@PBC) was added to the reaction flask containing 4-NP and NaBH₄ under constant stirring. Eventually, the intensity of the characteristic peaks at 400 nm was diminished; in contrast, the absorbance corresponding to the 4-aminophenolate ion at 297 nm increased and became colorless, indicating the successful reduction. Specifically, Ag@PBC 500 exhibited high catalytic activity compared to the other three catalysts. The reaction was completed within 3 minutes, Ag@PBC 300 and Ag@PBC 600 achieved it in 5 minutes, and Ag@PBC 400 achieved a complete reduction in 4 minutes even at a 1 : 1 mole ratio as shown in Fig. 10. Furthermore, the reduction reaction was evaluated at higher concentrations using different molar ratios of 4-NP to NaBH₄ (1 : 5, 1 : 10, 1 : 25, 1 : 50, 1 : 75, and 1 : 100). Ag@PBC 500 showed the highest catalytic efficiency with the 400 nm absorption band disappearing within seconds across all ratios. Ag@PBC 400 exhibited intermediate activity, whereas Ag@PBC 300 and Ag@PBC 600 had much slower kinetics, retaining the 400 nm feature for an extended period as shown in Fig. S9–S12.

Fig. 9 UV-Vis absorption spectra of 4-nitrophenol (4-NP) and 4-nitrophenolate ion.

Fig. 10 UV-Vis absorption spectra showing the reduction of 4-nitrophenol over time in the presence of different catalysts (a) Ag@PBC 300, (b) Ag@PBC 400, (c) Ag@PBC 500 and (d) Ag@PBC 600.

4.2. Dye reduction (MO and CR)

The presence of azo dyes in wastewater causes various hazards to aquatic organisms and human health. Auspiciously, Ag@PBC showed high catalytic behavior for the reductive degradation of these dye pollutants. The reaction process was monitored via UV-vis spectroscopy. The MO and CR peaks at around 464 nm and 498 nm, respectively, remained almost unchanged in the presence of NaBH₄, revealing that the reduction of MO and CR necessitates a catalyst.⁵⁵ Upon the addition of 1 mg of catalyst (Ag@PBC), the gradual decrease in absorbance at characteristic wavelengths (MO-464 and CR-498) confirmed the degradation of the dyes. The reduction efficiency was evaluated at three different dye concentrations (25, 50 and 100 ppm), shown in Fig. 11 and Fig. S13–S15. At 25 ppm, both Ag@PBC 300 and Ag@PBC 600 show the reduction of the dyes within 60 seconds. Conversely, Ag@PBC 400 and Ag@PBC 500 exhibit high catalytic activity. The reduction occurs within 45 seconds and 30 seconds, respectively. As the concentration increased to 50 ppm, the reaction time proportionally increased; Ag@PBC 300 and Ag@PBC 600 completed the reduction reaction within 120 seconds, while Ag@PBC 400 completed it within 90 seconds and Ag@PBC 500 showed efficient reduction within 60 seconds. At 100 ppm, a more significant decrease in reduction time was observed due to increased dye concentration; Ag@PBC 300 and Ag@PBC 600 completed the degradation of MO within 200 seconds and CR within 180 seconds. Ag@PBC 400 reduced MO within 180 seconds and CR within 150 seconds, while the Ag@PBC 500 showed a high catalytic activity and the reduction reaction was completed within 120 seconds for both MO and CR. The results show that the increase in dye concentration decreases the reduction rate. In addition, the practical efficiency of the Ag@PBC 400 °C and Ag@PBC 500 composites under environmentally relevant conditions, a mixed pollutant (4-NP, MO and CR) solution prepared to represent aquatic wastewater, was complex. As shown in Fig. 12, Ag@PBC 500 exhibited high catalytic activity, and the reduction reaction was completed within 6 minutes, whereas Ag@PBC 400 required 8 minutes for complete reduction. The effective catalytic reduction of all three pollutants in the mixture demonstrates the robust and versatile catalyst efficiency of the Ag@PBC composites under environmental conditions, reinforcing their potential application in wastewater treatment.

Fig. 11 Time-dependence of absorbance showing the reduction of CR and MO at different concentrations of 25 ppm (a) and (d), 50 ppm (b) and (e) and 100 ppm (c) and (f) catalyzed by Ag@PBC 500.

Fig. 12 Time-dependence of absorbance showing the reduction of a mixture of pollutants catalyzed by (a) Ag@PBC 400 and (b) Ag@PBC 500.

Among all catalysts, the Ag@PBC 500 composite shows effective catalytic reduction across all pollutants due to its well-defined porous structure, enhanced surface area, and silver nanoparticles facilitating the electron transfer from BH₄⁻ to dye molecules (Fig. 13). In contrast, Ag@PBC 300 and Ag@PBC 600 exhibit low catalytic activity due to less developed porosity at low pyrolysis temperature (300 °C) and possible structural degradation at high pyrolysis temperature (600 °C), both of which hinder efficient adsorption and reduction.

Fig. 13 The plot of ln(A_t/A₀) vs. time (t) for the 4-NP reduction.

4.3. Reaction kinetics

To elucidate the reaction mechanism, the kinetics of 4-NP and dye reduction using Ag@PBC as a heterogeneous catalyst have also been investigated. However, the excess concentration of NaBH₄ allows the reaction model to be pseudo-first order as given in eqn (1).

ln(A_t/A₀) = ln(C_t/C₀) = k_appt (1)

where C₀ and C_t are the initial and time-dependent concentration of 4-NP, respectively, A₀ and A_t correspond to the initial and time-dependent absorbance values and k_app is the apparent rate constant. Then, the rate constant for the catalytic reduction of 4-NP was evaluated at different molar ratios (1 : 1 to 1 : 100). The rate constant (k_app) increased with higher NaBH₄ concentration. For Ag@PBC 300, the k_app values vary from 0.1639 min⁻¹ (1 : 1) to 1.5002 min⁻¹ (1 : 100), while Ag@PBC 400 shows enhanced catalytic activity and the k_app values vary from 0.2358 min⁻¹ (1 : 1) to 2 min⁻¹ (1 : 100). A further improvement has been observed for Ag@PBC 500 where the k_app values are varying from 0.3314 min⁻¹ (1 : 1) to 3 min⁻¹ (1 : 100) indicating its higher catalytic activity towards the reduction of 4-NP. However, Ag@PBC 600 shows comparatively lower activity, with rate constants varying from 0.1921 min⁻¹ (1 : 1) to 1.5002 min⁻¹ (1 : 100), which may be attributed to structural modifications or the loss of active sites at higher pyrolysis temperature. The catalytic efficiency of the synthesized Ag@PBC composites for reduction of 4-nitrophenol (4-NP) using NaBH₄ was systematically compared with previously reported biochar and carbon-supported noble metal catalysts [Table 2 (ref. 56–63)]. Conventional catalysts such as nZVI–FBC (0.251 min⁻¹), Ag@CZ-TEB (1.194 min⁻¹), FC-900 (0.42 min⁻¹), and biogenic Ag–silicalite-1 (0.33 min⁻¹) indicate moderate activities, while noble metal-based hybrids like Au@BC-Fe₃O₄ (0.72 min⁻¹) and Pd@Fe₃O₄/biochar (1.47 min⁻¹) exhibit relatively enhanced kinetics. Although the bimetallic biochar@Cu–Ni catalyst achieved a high k_app of 6.81 min⁻¹, its synthesis is more complex and costly. In contrast, our Ag@PBC composites show excellent catalytic activity dependent on pyrolysis temperature and NaBH₄ : 4-NP molar ratio. The k_app values increased with higher NaBH₄ dosage, indicating the availability of more reducing equivalents. Among all, Ag@PBC 500 exhibited the highest catalytic activity (k_app = 3.0000 min⁻¹ at 100 : 1 molar ratio), outperforming most reported Ag–biochar and Ag–carbon-based catalysts. In addition to that, the k_app values for azo dyes using different catalysts and at different concentrations were calculated: for 25 ppm concentration, 1, 1.33, 2 and 1 min⁻¹ (both CR and MO) respectively; for 50 ppm concentration 0.4878, 0.5578, 1 and 0.4476 min⁻¹ (CR), 0.4455, 0.6629, 1 and 0.4468 min⁻¹ (MO) respectively, and for 100 ppm, 0.3272, 0.2854, 0.4502 and 0.2773 min⁻¹ (CR), 0.2764, 0.3199, 0.4250 and 0.2759 min⁻¹ (MO) respectively. Overall, the results confirm that Ag@PBC 500 shows the most efficient catalytic activity among the composites and the reduction rate is influenced by pollutant concentration and the reaction kinetics is higher for lower concentrations. The catalytic performance of the Ag@PBC composites in the reduction of different pollutants, including reduction time and rate constants, is given in Table 3.

Table 2 Comparison of the apparent rate constants (k_app) for the catalytic reduction of 4-nitrophenol (4-NP) using the reported catalysts and Ag@PBC catalysts prepared in this work at different NaBH₄ : 4-NP mole ratios

	Catalyst	Mole ratio	k app (min^−1)	Ref.
		NaBH4	4-NP
nZVI–FBC	140	1	0.251	20
Ag@CZ-TEB	100	1	1.194	56
FC 900	100	1	0.42	57
Biogenic Ag impregnated hollow silicalite-1	100	1	0.33	58
Au@BC–Fe3O4	100	1	0.72	59
BC–Fe3O4	100	1	0.276
Au@BC			0.282
Biochar@Cu–Ni	50	1	6.81	60
CuO@C	30	1	0.4086	61
SSBC-800	10	1	0.48	62
Pd@Fe3O4/biochar	5	1	1.47	63
Ag@PBC 300	1	1	0.1639
	5	1	0.1838
	10	1	0.3276
	25	1	0.3895
	50	1	0.6108
	75	1	1.0000
	100	1	1.5002
Ag@PBC 400	1	1	0.2359	This work
	5	1	0.2746
	10	1	0.2878
	25	1	0.4933
	50	1	1.0000
	75	1	1.3333
	100	1	2
Ag@PBC 500	1	1	0.3314
	5	1	0.4166
	10	1	0.4818
	25	1	1.0000
	50	1	1.3333
	75	1	2
	100	1	3
Ag@PBC 600	1	1	0.1922
	5	1	0.1841
	10	1	0.3267
	25	1	0.3460
	50	1	0.6194
	75	1	0.9154
	100	1	1.5002

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Table 3 Catalytic performance of the Ag@PBC composites in the reduction of different pollutants, showing reduction time and rate constants

Composites	Pollutants	Reduction time	k app (min^−1)
Ag@PBC 300	CR (25 ppm)	60 seconds	1
	CR (50 ppm)	120 seconds	0.4878
	CR (100 ppm)	180 seconds	0.3272
	MO (25 ppm)	60 seconds	1
	MO (50 ppm)	120 seconds	0.4455
	MO (100 ppm)	200 seconds	0.2764
Ag@PBC 400	CR (25 ppm)	45 seconds	1.33
	CR (50 ppm)	90 seconds	0.5578
	CR (100 ppm)	150 seconds	0.2773
	MO (25 ppm)	60 seconds	1.33
	MO (50 ppm)	120 seconds	0.3199
	MO (100 ppm)	180 seconds	0.2759
Ag@PBC 500	CR (25 ppm)	30 seconds	2
	CR (50 ppm)	60 seconds	1
	CR (100 ppm)	120 seconds	0.4502
	MO (25 ppm)	60 seconds	2
	MO (50 ppm)	120 seconds	0.6629
	MO (100 ppm)	120 seconds	0.4250
Ag@PBC 600	CR (25 ppm)	60 seconds	1
	CR (50 ppm)	120 seconds	0.4476
	CR (100 ppm)	180 seconds	0.2773
	MO (25 ppm)	60 seconds	1
	MO (50 ppm)	120 seconds	0.4468
	MO (100 ppm)	200 seconds	0.2759

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4.4. Effect of pH

The effect of pH on catalytic reduction of 4-nitrophenol and azo dyes was investigated in different reaction media. The reaction was slow at pH 4 (15.5 min) but proceeded much faster at pH 9 (6 min) and pH 11 (5.5 min). The reduced activity under acidic medium is due to the instability of NaBH₄, which undergoes rapid hydrolysis and hydrogen evolution, decreasing the availability of active hydride ions. Moreover, the protonation of –NO₂/–N=N– groups and surface –OH moieties under acidic conditions decreases the electron density of both the pollutants and catalyst, hindering adsorption and electron transfer. Consequently, electron transfer from BH₄⁻ to the substrate becomes inefficient, resulting in a slow reduction rate. In contrast, the rate of reaction is high under alkaline conditions arising from the formation of reactive nitrophenolate ions and the improved stability of BH₄⁻, which promotes faster electron transfer and rapid conversion and cleavage of –NO₂ and –N=N– bonds.⁶⁴ Overall, the optimal pH range for efficient reduction lies in the mildly alkaline region (pH 8–11), where the catalyst stability and electron transfer efficiency are maximized.

4.5. Mechanism

The catalytic reduction of 4-NP and azo dyes by using a heterogeneous catalyst could be summarized within three steps: adsorption of NaBH₄ and the pollutant, electron transfer from borohydride to the pollutant and desorption of the reduced product. The synergistic mechanism for enhanced catalytic reduction of pollutants through Ag@PBC composites is illustrated in Fig. 14. In the first step, pollutants and NaBH₄ adsorb onto the Ag@PBC composite surface. The biochar matrix has high surface area, porosity and an aromatic framework, which support the reactants adsorption and rapid electron transfer to active sites thereby increasing the apparent reaction rates. Its surface contains oxygen containing groups (–OH, C–O, C=O, COO⁻ and –COOH), which enhance the adsorption of both pollutant molecules and borohydride ions following the electron transfer.^65,66 In addition to that, hydroxyl groups increase the reactant adsorption through hydrogen bonding near the Ag active sites for efficient reduction. Also, the biochar carbon structure involves electron transfer from borohydride to the pollutant, which lowers the kinetic barrier and accelerates the reduction reaction.⁶⁷ At this stage, the synergistic effect of biochar and silver nanoparticles becomes significant. The Ag nanoparticles serve as active electron-transfer sites, while biochar not only stabilizes these nanoparticles but also in some cases, exhibits semiconducting behavior through its conjugated aromatic structures, which facilitates efficient electron transfer from donor (BH₄⁻) to acceptor (pollutant).^68,69 In 4-NP reduction, the Ag@PBC composite promotes electron transfer from BH₄⁻ ions to the nitro group (–NO₂), reducing it stepwise to hydroxylamine (–NHOH) and finally to 4-aminophenol (–NH₂) through a six electron transfer process.⁷⁰ In the case of azo dyes, the Ag@PBC composite promotes the cleavage of the azo bonds (–N=N–) through successive hydrogenation, yielding aromatic amines such as sodium 3,4-diaminonaphthalene-1-sulfonate, benzidine, and sulfanilic acid derivatives.^71,72 In the final step, the reduced products are desorbed from the composite surface and assimilated into water.⁷³ Schematic representation of the catalytic reduction mechanism of (a) 4-NP, (b) Congo red and (c) methyl orange by the Ag@PBC composite is shown in Fig. 15.

Fig. 14 Possible reduction mechanism of 4-NP and azo-dyes (MO and CR) by the Ag@PBC composite.

Fig. 15 Schematic representation of the catalytic reduction mechanism of (a) 4-NP, (b) Congo red and (c) methyl orange by the Ag@PBC composite.

5. Reusability

One of the salient features of heterogeneous catalysts is their easy recovery and reusability over multiple cycles without loss of activity. After the reduction reaction, the composites were allowed to settle and separated using centrifugation. Then the composites were washed thoroughly with deionized water and dried before reuse.^74,75 The reusability of the Ag@PBC composites was evaluated over ten consecutive cycles and the results are shown in Fig. S16. All four catalysts retained excellent catalytic activity during the initial cycles and maintained it nearly up to the seventh cycle. However, for Ag@PBC 300 and Ag@PBC 600, the catalytic activity was decreased slightly from eighth cycle. After the tenth cycle, both composites retained 94.5%. Similarly, Ag@PBC 400 and Ag@PBC 500 retained 96% and 98%, respectively, after ten cycles. This decrease in activity could be partial aggregation of silver nanoparticles, surface adsorption of pollutants or structural degradation of the biochar. Nevertheless, the FTIR spectra of the Ag@PBC composites (Fig. S17) indicate that there are no significant changes in the structural integrity of the biochar, suggesting that the biochar structure remains stable after repeated use. TEM analysis was carried out for the composite before and after the four cycles. The corresponding TEM images are presented in Fig. 16(a)–(c). Specifically, Fig. 16a shows the Ag@PBC composite before the reaction, Fig. 16b after the catalytic reaction, and Fig. 16c after the recovery process. As observed in Fig. 16(a)–(c), the Ag nanoparticles remain well dispersed on the biochar surface without aggregation, confirming the silver–biochar interaction and structural stability of the catalyst. Thus, the results clearly demonstrate the excellent recyclability of the Ag@PBC composites, which suggests the practical application of the composites in wastewater treatment.

Fig. 16 TEM micrograph of the composite (a) before and (b) after the reaction and (c) after the recovery process.

6. Challenges and future work

Ag@PBC composites demonstrate excellent catalytic efficiency and stability, but several challenges remain for their practical application. Factors such as temperature fluctuations, fouling by organic matter, variable ionic strength, suspended solids and pH variations can significantly affect catalytic activity and reusability. Additionally, issues like Ag⁺ leaching, difficulties in catalyst recovery, and inconsistent activity under complex water matrices, remain major obstacles in large scale applications. Comprehensive life cycle assessment and ecotoxicological studies are also required to ensure environmental safety. Future studies will focus on developing magnetically separable biochar-based catalysts on Borassus flabellifer biochar and validating the catalyst performance in continuous flow and variable temperature conditions to bridge the gap between laboratory and industrial waste water treatment.

7. Conclusion

In conclusion, a series of silver–biochar composites (Ag@PBC) were successfully prepared using Borassus flabellifer bark via controlled pyrolysis (300–600 °C) and chemical reduction of silver ions. TGA analysis indicated that the temperature varying from 300 to 600 °C facilitated efficient biomass carbonization, producing structurally stable biochar. The prepared composites were characterized using XRD, FTIR, UV-vis, SEM, TEM, XPS, and BET analyses, which revealed that increasing the pyrolysis temperature and incorporation of Ag nanoparticles led to enhanced surface area and porous structure. TEM analysis confirmed the uniform distribution of silver nanoparticles (5–15 nm) on the biochar surface. The catalytic activities of the catalysts (Ag@PBC 300, Ag@PBC 400, Ag@PBC 500 and Ag@PBC 600) were studied in the reduction of 4-NP and azo dyes (MO and CR) using sodium borohydride under mild conditions. Also, a mixed pollutant solution was prepared in groundwater to emulate aquatic environmental conditions. Among the four catalysts, Ag@PBC 500 exhibited the highest catalytic activity due to its robust porous structure and silver nanoparticle deposition, enabling efficient electron transfer from NaBH₄ to the pollutants. Notably, the composites effectively reduced the mixed pollutants within 6 minutes, highlighting their practical application in industrial wastewater treatment. Kinetic analysis confirmed the pseudo-first-order reaction behaviour, with the highest apparent rate constants observed for Ag@PBC 500. In addition, catalytic reduction proceeded efficiently in a mild alkaline medium, with an optimum pH range of 8–11. Furthermore, the composites demonstrated excellent reusability, maintaining over 94–98% catalytic efficiency after ten consecutive cycles. These findings highlight that Ag@PBC composites are an affordable, efficient and eco-friendly catalyst to treat industrial wastewater contaminated with persistent toxic pollutants such as dyes and nitroaromatic compounds.

Author contributions

Abraham Stuvart: writing – original draft, visualization, methodology, formal analysis, and conceptualization. Thesingu Rajan Arun: writing – review and editing, supervision, project administration, funding acquisition, formal analysis, and data curation. Natarajan Raman: supervision, resources, investigation, visualization, and validation.

Conflicts of 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 research article.

Data availability

Data will be made available on request.

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5nj03733f.

Acknowledgements

The authors express their gratitude to the Director, Dean and Head of the Department of Chemistry, SRM Institute of Science and Technology, Tiruchirappalli, for providing the necessary facilities to complete this work successfully. We acknowledge Nanotechnology Research Centre (NRC), SRMIST for providing the research facilities. The authors gratefully acknowledge the financial support from SRMIST-KTR for this study under Grant No. SRMIST-SERI 2024/174/20-342.

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