ABSTRACT
This study investigates the photocatalytic degradation of methylene blue (MB) using an Ag-Pterocarpus santilinoides extract biochar nanocomposites (AgPSBN) followed by characterization and antimicrobial screening of the biogenic photocatalyst. The silver biochar nanocomposite was synthesized by incorporating silver nanoparticles onto the surface of biochar through a facile, biogenic, safe, cost-effective and ecofriendly method. The photosynthesized AgPSBN was porous and spherical with a size of 27 nm range. The UV-spectroscopic analysis indicated spectra bands at 648 and 667 nm for biochar and nanocomposite, respectively. The low band gap energy of 2.0 and 1.8 eV for the biochar and nanocomposite, respectively, is an indication that they can be an effective photocatalyst for the degradation of MB and for other energy applications. The percentage removal efficiency of 96.33% indicates high photodegradation ability which remained fairly constant (75%) after five cycle reuse indicating stability of the nanocomposite. The rate constant was evaluated to be 0.008 min−1. The nanocomposite indicated high inhibition zone diameter for Salmonella, E. coli, Klebsiella, and Staphylococcus aureus with inhibition zone diameters of 15, 12, 10, and 8 mm, respectively. The results strongly suggest the nanocomposite to be an effective environmental decontaminant of dyes as well as microbes.
HIGHLIGHTS
Ecofriendly silver@pterocarpus santilinoides biochar-organic–inorganic hybrid nanocomposites were fabricated.
Nanocomposite was crystalline, porous and spherical with a size of 27 nm range.
Photocatalytic study indicated a 96.33% decimation of MB.
Antimicrobial effectiveness shows high zones of microbial inhibition.
Reusability of the nanocomposite indicated 75% stability after five cycles.
INTRODUCTION
Photocatalytic degradation is currently at the forefront of research as an advanced and environmentally friendly approach for the removal of organic pollutants from contaminated water and other matrixes. Among various photocatalysts, nanocomposites have gained significant attention due to their enhanced catalytic properties and synergistic effects (Mohammed et al. 2021; Balaji et al. 2023; Payel & Debajyoti 2023). One such promising nanocomposite is the silver biochar nanocomposite, which combines the photocatalytic activity of silver nanoparticles with the adsorption capacity of biochar. By combining the unique properties of silver nanoparticles (AgNPs) with Pterocarpus santilinoides leaves and biochar, a synergistic effect is achieved, leading to enhanced photocatalytic and antimicrobial capabilities (Nworie et al. 2023).
Methylene blue (MB) is a commonly used dye in industries such as textiles, printing, and paper production. However, its release into water bodies poses a severe threat to the environment and human health due to its toxicity and carcinogenic properties (Ahmed et al. 2023; Mustafa et al. 2023). Therefore, the development of efficient and cost-effective methods for its removal is of utmost importance.
P. santilinoides, commonly known as African padauk, is a tree species found in tropical regions. Pterocarpus is a genus of flowering plants in the family Fabaceae (legume family), and it includes several species, such as Pterocarpus santalinus (Indian redwood or red sandalwood) and Pterocarpus indicus (Amboyna wood). These species are known for their timber, medicinal properties, and cultural significance in various regions. Extracts from various parts of the plant, including the leaves, bark, and heartwood, are believed to have potential health benefits, such as anti-inflammatory, antioxidant, antimicrobial, and wound-healing properties (Ihedioha et al. 2019). Biochar, a carbon-rich material produced through the pyrolysis of biomass, has gained prominence as an ecofriendly and sustainable material. Biochar has a highly porous structure and high carbon content making it a good candidate for the sorption of contaminants and in inactivation of microorganisms (Abdelazeem et al. 2022).
Silver nanoparticles (AgNPs) possess unique optical, electrical, and catalytic properties, making them excellent candidates for photocatalytic applications and antimicrobial materials (Abdelazeem et al. 2022). Several studies have implicated silver nanoparticles to effectively generate reactive oxygen species (ROS) upon exposure to light, which can oxidize and degrade organic pollutants (Singh & Dhaliwal 2020). However, the aggregation and limited stability of AgNPs restrict their practical application. To overcome these limitations, researchers have incorporated AgNPs into biochar, a carbonaceous material derived from biomass pyrolysis. Biochar possesses a high surface area, porous structure, and strong adsorption capacity, which can immobilize AgNPs, facilitate the photocatalytic degradation process and enhance antimicrobial properties (Abdelazeem et al. 2022; Nworie et al. 2023). The combination of silver, biochar, and leaf extract creates a powerful composite material with enhanced photocatalytic and antimicrobial activity. The resulting silver biochar nanocomposite combines the advantages of both materials (silver nanoparticle and biochar), leading to improved stability, increased catalytic efficiency, increased antimicrobial potency, and enhanced dye adsorption capacity. The incorporation of silver nanoparticles into biochar matrices offers several advantages. Firstly, the biochar acts as a stabilizing agent for the silver nanoparticles, preventing their aggregation and ensuring a sustained release of silver ions over time. This sustained release is crucial for long-lasting antimicrobial effects, a quality currently sought in drug industries. Secondly, biochar provides a stable and environmentally friendly support material for silver nanoparticles, allowing their application in various fields, including water treatment, health care, food packaging, wound healing, environmental remediation, and agriculture (Mohammed et al. 2021; Balaji et al. 2023). In the healthcare sector, these nanocomposites can be used in wound dressings, medical devices, and coatings to prevent infections and promote faster healing. In environmental applications, they can be employed for water purification and filtration, effectively removing harmful bacteria and pathogens (Abdelazeem et al. 2022). Additionally, incorporating silver biochar nanocomposites into food packaging materials can extend the shelf life of perishable products by inhibiting the growth of spoilage microorganisms.
Silver biochar nanocomposites have gained significant attention in recent years due to their remarkable antimicrobial properties. The antimicrobial activities of silver biochar nanocomposites arise from the synergistic effects of silver nanoparticles and the porous structure of biochar (Abdelazeem et al. 2022). Silver nanoparticles exhibit excellent antimicrobial properties by releasing silver ions, which have the ability to disrupt bacterial cell membranes and inhibit the growth of a wide range of microorganisms. Biochar, on the other hand, provides a high surface area and a porous structure that enhances the contact between the nanocomposites and microorganisms, facilitating antimicrobial action. The antimicrobial activities of silver biochar nanocomposites have been extensively studied against a wide range of microorganisms, including bacteria, fungi, and viruses (Abdelazeem et al. 2022). Numerous studies have reported the effective inhibition and killing of pathogenic bacteria such as Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and Salmonella spp (Abdelazeem et al. 2022). The nanocomposites have also demonstrated strong antifungal properties against species such as Candida albicans and Aspergillus niger (Abdelazeem et al. 2022). Moreover, recent research has explored the antiviral potential of silver biochar nanocomposites, showing promising results in inhibiting the replication of viruses like influenza and herpes simplex (Akbarzadeh et al. 2018).
In this study, the synthesis of the nanocomposite, structural and morphological properties, photocatalytic performance and antimicrobial properties were evaluated. The effects of various parameters, such as catalyst time of contact and initial dye concentration, were systematically studied to optimize the degradation efficiency.
METHODS
Instrumentation
The UV-Vis spectra were obtained using a Genesis 10S UV-Vis spectrophotometer. A Bruker® D8 Discover x-ray diffractometer, equipped with a Lynx Eye detector, under Cu-Ka radiation (λ = 1.5406 Å) radiation at 40 kV and at 30 mA) was used for the determination of the crystalline nature of the biochar and nanocomposite and diffraction recorded with 2θ = 10 to 70. Phenomprox by Phenom World Eindhoven, Netherlands was used to determine the structural morphology and elemental composition of the nanocomposite and biochar (SEM/EDX analysis). A Cary 630 Agilent Technologies, USA Fourier transform infrared spectrophotometer (Shimadzu, Japan) in the range of 4,000–400 cm−1 in KBr using nujol mull as the mulling agent was used for functional group determination. All drying and shakings were effected using oven (model DHG) rotary shaker (RF-12 Remi equipment), respectively. Autoclave (Alpha Laboratory, Ltd) model NL280-A, Pressure Cooker (Crown Star Atsago, India Instruments Pvt Ltd, India and water bath (helmreasinn multipurpose) model: DK420 were used for the antimicrobial experiment.
Reagents
The following analytical grade reagents were obtained from Merck Germany and used without further purification; nitric acid (HNO3), MB dye, sodium hydroxides (NaOH), aqueous ammonia (NH3), dimethylsulfoxide, and hydrochloric acid (HCl). Nutrient broth and Mueller Hinton agar were sourced from TM-media laboratory PVT ltd.
Preparation of P. santalinoides leaf extract
P. santalinoides leaves were washed, dried for 7 days under the sun and ground into powder. Exactly 5 g of the powdered leaves were dissolved in 100 cm3 of distilled water and then heated with stirring at 80 °C for 30 min. The solution was then filtered and the filtrate was kept at 4 °C for further use.
Preparation of P. santalinoides leaf biochar
The P. santilinoides leaves were collected, cleaned, washed severally with deionized water and dried under the sun for 7 days. The dried leaves were then kept at a temperature of 60 °C for 3 h in an oven. The oven-dried leaves (500 g) were carbonized in an N2 environment at the temperature of 500 °C for 2 h. The carbonized biochar product was then ground into fine powder and kept for further analysis in an airtight container.
Synthesis of Ag-P. santilinoides extract biochar nanocomposites (AgPSBN)
Exactly 0.79 g of pulverized P. santilinoides biochar was added to 100 cm3 of distilled water, followed by the addition of 0.015 g of AgNO3 to the biochar dispersion and the mixture shaken vigorously for 30 min. Thereafter 10 cm3 of P. santilinoides extract was added to the solution, stirred and heated at 80 °C to reduce the silver ions on the surface of the biochar. The nanocomposite formed was separated by centrifugation, washed three times with distilled water and ethanol and then dried in an oven at 60 °C for 24 h.
Photocatalytic degradation of MB under different concentrations
Variation of time of photocatalytic activity (kinetic studies)
The effect of time on the photocatalytic degradation of MB was executed using five separate extraction bottles. Exactly 5 mg (0.005 g) of nanocomposite and 10 cm3 of 25 ppm of MB in five separate extraction bottles were prepared in the dark and allowed to equilibrate for 30 min. The different extraction bottles labeled A to E were allowed under the sunlight and removed at intervals of 20, 40, 60, 80, and 100 min, respectively. Thereafter, 2 cm3 of the aliquots were removed and centrifuged at 17,000 rpm for 2 min and the absorbance of the supernatant MB was measured at 640 nm in a quartz cuvette.
Desorption of MB from nanocomposite and reusability
Exactly 10 cm3 of different concentrations ranging from 0.10 to 0.001 M of HCl was added into three different extraction bottles containing 0.05 g of nanocomposites obtained after the photocatalytic experiment. The three sample bottles were agitated for 30 min, then filtered and the UV of the supernatant was taken at 640 nm against blank. The recovered nanocomposite was dried and reused for the removal of MB. This was repeated five times and the UV at each cycle extraction was recorded at 640 nm against blank.
Antimicrobial screening of the biochar and nanocomposite
Organisms studied are biologically relevant in clinical fields and include S. aureus, Salmonella sp., Klebsiella, and Escherichia coli. The clinical isolates were sourced from the Federal Teaching Hospital, Abakaliki, Nigeria and transported using nutrient broth to the ultra-modern laboratory of the Department of Applied Microbiology, Ebonyi State University, Abakaliki. The antimicrobial inhibitory test was performed on the biochar and nanocomposites against the test organisms S. aureus, Salmonella sp, Klebsiella and Escherichia coli as described elsewhere with little modifications of first dissolving the biochar and nanocomposites in dimethylsulfoxide (Nworie et al. 2018; Mohammed et al. 2021; Balaji et al. 2023). The culture of the test organisms was made in nutrient broth, incubated for 24 h and adjusted to the MCFarland turbidity standard. Blanks which act as control were prepared from a single antibiotic disc impregnated with ampiclox aseptically deposited on the Mueller Hinton agar plates. The culture of the test organisms and control were incubated at 370 °C for 18–24 h. The inhibition zone diameter was evaluated.
RESULTS AND DISCUSSION
(a) Fourier infrared (FTIR) spectroscopy analysis of Pterocarpus santanoloides biochar. (b) Fourier infrared (FTIR) spectroscopy analysis of nanocomposite. (c) Fourier infrared (FTIR) spectroscopy analysis of MB nanocomposite.
(a) Fourier infrared (FTIR) spectroscopy analysis of Pterocarpus santanoloides biochar. (b) Fourier infrared (FTIR) spectroscopy analysis of nanocomposite. (c) Fourier infrared (FTIR) spectroscopy analysis of MB nanocomposite.
UV-visible spectroscopy
UV-Visible spectroscopy of (a) biochar (b) nanocomposite. (c,d) Band gap energy of (c) biochar (d) nanocomposite.
UV-Visible spectroscopy of (a) biochar (b) nanocomposite. (c,d) Band gap energy of (c) biochar (d) nanocomposite.
XRD of biochar and nanocomposite

SEM and EDX analysis
SEM micrograph of (a) biochar, (b) nanocomposite, and (c) MB nanocomposite.
(a) EDX analysis of biochar. (b) EDX analysis of nanocomposite. (c) EDX analysis of MB nanocomposite.
(a) EDX analysis of biochar. (b) EDX analysis of nanocomposite. (c) EDX analysis of MB nanocomposite.
Photocatalytic studies on the nanocomposite
(a) UV-Vis degradation spectra of MB at different time intervals (a) (0.001 M MB). (b) Kinetics of degradation of MB (0.001 M MB). (c) UV-Vis degradation spectra of MB at different concentrations. (d) Reusability of the nancomposite.
(a) UV-Vis degradation spectra of MB at different time intervals (a) (0.001 M MB). (b) Kinetics of degradation of MB (0.001 M MB). (c) UV-Vis degradation spectra of MB at different concentrations. (d) Reusability of the nancomposite.
The photocatalytic ability of the biosynthesized nanocomposite for the photodegradation of MB was evaluated under natural sunlight using varied concentrations of MB (0.01–0.0001 M). The Ag nanocomposite (5 mg) was allowed under intense stirring in the dark to attain equilibrium and to encourage the migration of the MB molecules into the nanocomposite matrix before introducing it into the natural sunlight zone for the photocatalytic process. Thereafter, the MB concentration was measured at 640 nm. The maximum photocatalytic efficiency of 93.909% was observed at a concentration of 0.001 M (Figure 6(c)). The percentage of photocatalytic activity decreased at higher MB concentrations as shown in Figure 6(c) probably because of the intense color of the MB which inhibited penetration of the sunlight into the MB Ag nanocomposite mixture. The percentage photocatalytic degradation was however close and indicated some degree of synergy between the biochar and AgNPs that facilitated the ultraviolet light attracting ability of the Ag nanocomposite, a consequence of surface Plasmon resonance phenomenon and the biochar graphitic nature (Abdelazeem et al. 2022). Consequently, ROS are generated as interfacial charge separation gets reduced and recombination electron-hole pairs quenched. The photocatalytic activity of the biochar alone was 10% and the MB was 7.20% further supporting the fact that the photocatalytic activity emanated from the Ag nanocomposite
Reusability of the Ag nanocomposite after desorption with 0.01 M HNO3 indicated that the efficiency decreased continuously from 89.50 to 72% after five cycles of reuse as shown in Figure 6(d). This indicates that the product is photostable and environmentally friendly.
Mechanism of MB removal through photocatalytic adsorption
Comparing the photodegradation ability of the photofabricated nanocomposite with other photocatalysts in some research work indicates that the AgPSBN is highly effective for the photodegradation of organic pollutants such as MB and as such can be used for environmental remediation. The comparison is illustrated in Table 1.
Comparison between the degradation capacity/ability of the nanocomposite and other fabricated nanocomposites
Catalyst . | Photodegradation capacity (%) . | Dye conc (mg/L) . | Time (min) . | References . |
---|---|---|---|---|
AgNPs | 82.8 | 60 | 180 | Fairuzi et al. (2018) |
Ag/ZnO | 81.2 | 25 | 240 | Singh & Dhaliwal (2020) |
Ag/ZnO nanocomposite | 94.3 | 10 | 120 | Abdel Messih et al. (2019) |
Ag@biochar nanocomposite | 88.4 | 25 | 75 | Abdelazeem et al. (2022) |
Ag-biochar nanocomposite (AgPSBN) | 96.33 | 25 | 20 | This work |
Catalyst . | Photodegradation capacity (%) . | Dye conc (mg/L) . | Time (min) . | References . |
---|---|---|---|---|
AgNPs | 82.8 | 60 | 180 | Fairuzi et al. (2018) |
Ag/ZnO | 81.2 | 25 | 240 | Singh & Dhaliwal (2020) |
Ag/ZnO nanocomposite | 94.3 | 10 | 120 | Abdel Messih et al. (2019) |
Ag@biochar nanocomposite | 88.4 | 25 | 75 | Abdelazeem et al. (2022) |
Ag-biochar nanocomposite (AgPSBN) | 96.33 | 25 | 20 | This work |
Antimicrobial evaluation of the nanocomposite
The results of antimicrobial studies, particularly the measurement of microbial inhibition zone diameter, provide valuable information about the effectiveness of antimicrobial agents against specific bacteria. The studied microorganisms are relevant; Salmonella is a common pathogen associated with foodborne illnesses; E. coli is a commonly studied bacterium due to its prevalence in food contamination and waterborne diseases; Klebsiella species can cause various infections, including pneumonia and urinary tract infections; S. aureus is a common bacterium responsible for skin infections and can also cause more severe conditions.
Comparison between the antimicrobial efficacy of the nanocomposite and other nanocomposites
Sample . | Concentration (mg/mL−1) . | Bacterial strain . | Microbial zone of inhibition (mm) . | References . |
---|---|---|---|---|
Ag/GO nanocomposite | 1 | Staphylococcus aureus | 15 | Jeronsia et al. (2020) |
E. coli | 19 | |||
Copper/silver titanium oxide nanocomposite (Cu–Ag–TiO2) | 0.5 | Staphylococcus | 21 | Ghosh et al. (2021) |
E.coli | 16 | |||
Ag@biochar nanocomposite | 1 | Staphylococcus | Resistant | Abdelazeem et al. (2022) |
E. coli | No growth | |||
Klebsiella | 18 | |||
AgNp embedded guar gum/gelatin nanocomposite | 0.5 | Staphylococcus | 13 | Khan et al. (2020) |
E. coli | 12.5 | |||
Pseudomonia aeuroginosa | 12 | |||
AgPSBN nanocomposite | 0.5 | Staphylococcus | 08 | This work |
E. coli | 12 | |||
Klebsiella | 10 | |||
Salmonella | 15 |
Sample . | Concentration (mg/mL−1) . | Bacterial strain . | Microbial zone of inhibition (mm) . | References . |
---|---|---|---|---|
Ag/GO nanocomposite | 1 | Staphylococcus aureus | 15 | Jeronsia et al. (2020) |
E. coli | 19 | |||
Copper/silver titanium oxide nanocomposite (Cu–Ag–TiO2) | 0.5 | Staphylococcus | 21 | Ghosh et al. (2021) |
E.coli | 16 | |||
Ag@biochar nanocomposite | 1 | Staphylococcus | Resistant | Abdelazeem et al. (2022) |
E. coli | No growth | |||
Klebsiella | 18 | |||
AgNp embedded guar gum/gelatin nanocomposite | 0.5 | Staphylococcus | 13 | Khan et al. (2020) |
E. coli | 12.5 | |||
Pseudomonia aeuroginosa | 12 | |||
AgPSBN nanocomposite | 0.5 | Staphylococcus | 08 | This work |
E. coli | 12 | |||
Klebsiella | 10 | |||
Salmonella | 15 |
CONCLUSIONS
The photosynthesized AgPSBN was showcased for the first time. The phytochemicals present in the P. saintalinoides were implicated to be responsible for the bioreduction of the silver ions into AgNPs decorated on the biochar surface. The AgPSBN was spherical with a size of 27 nm range. The low band gap energy of the nanocomposite of 1.8 eV is an indication that it is an effective photocatalyst for the degradation of MB and for other energy applications. The percentage removal efficiency of 96.33% indicates high photodegradation ability which remained fairly constant (75%) after five cycle reuse indicating stability of the nanocomposite. The rate constant was evaluated to be 0.008 min−1 and compares favorably with similar works. The nanocomposite indicated high inhibition for Salmonella, E.coli, Klebsiella, and S. aureus with inhibition zone diameters of 15, 12, 10, and 8 mm, respectively. The results strongly suggest the nanocomposite to be an effective environmental decontaminant of dyes as well as microbes.
ACKNOWLEDGEMENTS
The authors acknowledge Dr Boniface Oke of the microbiology department at Ebonyi State University, Abakaliki for helping with antimicrobial analysis
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interest.