Silver Loading Onto Rice Husk Biochar Using Avocado Seed Extract Assisted By LED Light: Catalyst and Antibacterial Capabilities.

In this study, we present a novel approach for the green synthesis of biochar impregnated with silver nanoparticles (AgNPs) using agricultural waste, specifically avocado seed extract, and blue LED light. The goal of this research was to produce an environmentally friendly material that exhibits both catalytic and antibacterial properties, with potential applications in wastewater treatment, environmental remediation, and healthcare. Biochar, a carbon-rich material derived from biomass, was selected for its high porosity and ability to support metal nanoparticles, which in turn can enhance its functionality. Silver, known for its antibacterial and catalytic efficiency, was chosen as the metal for nanoparticle formation.

The synthesis method developed in this study offers a sustainable alternative to conventional chemical reduction techniques, which often rely on toxic chemicals and require high energy inputs. By utilizing an avocado seed extract as a natural reducing agent and activating the reaction with blue LED light, we successfully impregnated biochar with well-dispersed AgNPs. This eco-friendly approach aligns with the increasing demand for green chemistry solutions in material synthesis and provides a scalable method for future industrial applications.

The process begins with the production of biochar from rice husk, a lignocellulosic biomass waste commonly available in agricultural settings. The rice husk was subjected to pyrolysis, a thermochemical process that transforms the biomass into porous biochar. This biochar serves as the support matrix for silver nanoparticles. By varying the concentration of the avocado seed extract, we were able to control the size and distribution of the silver nanoparticles on the biochar surface. This aspect is critical, as the size of the nanoparticles directly influences both the catalytic activity and the antibacterial effectiveness of the material.

Once the biochar was prepared, it was impregnated with silver nitrate solution, and the reduction of silver ions to form nanoparticles was achieved using the avocado seed extract. This method leverages the natural phytochemicals in the extract, which act as reducing and capping agents, to stabilize the nanoparticles and prevent agglomeration. Blue LED light was employed to assist the reduction process, ensuring a homogeneous distribution of nanoparticles across the biochar surface. This light-assisted synthesis method is not only energy-efficient but also promotes the formation of smaller, well-dispersed nanoparticles, which are advantageous for both catalysis and antibacterial applications.

The catalytic potential of the silver-loaded biochar was assessed by testing its ability to reduce methyl orange, a common azo dye used in industrial applications. The reduction of methyl orange was chosen as a model reaction to evaluate the effectiveness of the biochar-AgNP composite as a catalyst. The results demonstrate that the biochar impregnated with smaller nanoparticles, specifically those synthesized with the lowest concentration of avocado seed extract, exhibited the highest catalytic activity. Nearly 95% of the methyl orange was reduced within five minutes, indicating that the material is highly effective in promoting the breakdown of azo dyes. This finding highlights the potential of the synthesized biochar as a sustainable catalyst for wastewater treatment, where efficient dye removal is crucial.

In addition to its catalytic properties, the biochar-AgNP composite was tested for antibacterial activity against three bacterial strains: Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa. These strains were selected to represent both Gram-positive and Gram-negative bacteria, providing a comprehensive assessment of the material's antibacterial capabilities. The results showed a significant antibacterial effect against Staphylococcus aureus, a Gram-positive bacterium, with an inhibition zone measuring 19.64 ± 0.247 mm for the sample synthesized with the lowest concentration of avocado seed extract. This is particularly noteworthy, as Staphylococcus aureus is a common pathogen responsible for various infections, and the ability to inhibit its growth suggests potential medical applications for the biochar-AgNP composite.

However, the material did not exhibit antibacterial activity against the Gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa. This outcome can be attributed to the structural differences between Gram-positive and Gram-negative bacteria. Gram-negative bacteria possess an additional outer membrane that acts as a barrier, making them more resistant to silver nanoparticles. The selective antibacterial activity observed in this study suggests that the biochar-AgNP composite may be more suitable for targeting Gram-positive pathogens, though further research is needed to enhance its effectiveness against Gram-negative bacteria.

The ability to control the size of the nanoparticles by adjusting the concentration of the avocado seed extract was a key aspect of this study. Smaller nanoparticles offer a larger surface area for interaction with bacterial cells and catalytic reactions, making them more effective in both applications. The sample with the smallest AgNPs, produced using 0.15 g of avocado seed extract in 50 mL of solution, consistently outperformed the other samples in terms of both catalytic and antibacterial activity. This underscores the importance of nanoparticle size and dispersion in determining the functionality of the material.

In addition to the size and dispersion of the nanoparticles, the structural integrity of the biochar itself was preserved throughout the synthesis process. The porosity and surface area of the biochar remained largely unaffected by the impregnation of silver nanoparticles, which is essential for maintaining its adsorptive properties in catalytic and antibacterial applications. The textural analysis confirmed that the biochar retained its mesoporous structure, with a specific surface area of 5.97 m²/g for the sample with the smallest nanoparticles. This ensures that the material can effectively interact with both contaminants and bacteria in practical applications.

The silver nanoparticles were thoroughly characterized using a range of analytical techniques, including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). SEM images revealed the porous structure of the biochar and confirmed the uniform dispersion of silver nanoparticles on its surface. EDS analysis further validated the presence of silver, along with other elements naturally occurring in rice husk biochar, such as silicon, calcium, and potassium. XRD spectra indicated the successful reduction of silver ions to zero-valent silver, with characteristic diffraction peaks corresponding to crystalline silver nanoparticles. These results confirm the effectiveness of the green synthesis method in producing well-defined AgNPs on biochar.

One of the key advantages of this synthesis approach is its scalability and environmental sustainability. By utilizing agricultural waste as both the source of biomass for biochar and the reducing agent for silver nanoparticles, we have developed a process that minimizes waste and reduces the need for harmful chemicals. The use of LED light as an energy source for nanoparticle formation further enhances the eco-friendliness of the method, as it reduces energy consumption compared to traditional thermal methods. This green synthesis technique could be applied to other metals, such as gold, copper, and zinc, broadening the potential applications of biochar in catalysis, environmental remediation, and medicine.

The findings of this study suggest several promising avenues for future research. While the biochar-AgNP composite demonstrated significant antibacterial activity against Gram-positive bacteria, further work is needed to improve its efficacy against Gram-negative strains. This could involve modifying the synthesis process to produce smaller nanoparticles or incorporating other metals with known antibacterial properties. Additionally, the long-term stability and reusability of the biochar-AgNP composite should be investigated, particularly in real-world applications such as wastewater treatment and medical devices.

In conclusion, we have developed a novel, green synthesis method for producing biochar impregnated with silver nanoparticles using avocado seed extract and blue LED light. The resulting material exhibits excellent catalytic activity in the reduction of methyl orange and significant antibacterial activity against Staphylococcus aureus. This eco-friendly approach provides a sustainable alternative to conventional synthesis methods and offers potential applications in a range of industries, including wastewater treatment, environmental remediation, and healthcare. The ability to control nanoparticle size and dispersion through simple adjustments in the synthesis process further enhances the versatility of the biochar-AgNP composite, making it a promising candidate for future development in sustainable material science.

Reference:

Moreno, E., Gualle, A., Vizuete, K., Debut, A., Orejuela, L., & Ponce, S. (2024). Engineered Biochar: Novel Silver Loading onto Biochar Using Agri-waste Extract Assisted by LED Light for Catalyst and Antibacterial Capabilities. Waste and Biomass Valorization, 1-10.