Green synthesis of microalgal biomass-silver nanoparticle composite showing antimicrobial activity and heterogenous catalysis of nitrophenol reduction.

In this study, we have demonstrated an integrated approach for utilization of microalga Scenedesmus sp. for fabrication of catalytic and antimicrobial silver nanoparticle composite. The algal biomass was harvested from an open raceway pond of 30,000 L scale used for CO₂ sequestration. The dried biomass served as a green, nontoxic, reducing and immobilizing agent for synthesis of silver nanoparticles, producing biomass-silver nanoparticle composite. ICP-OES was used to monitor the uptake of silver ions by biomass and subsequent formation of nanoparticles. The composite was calcined at 400 °C to fix the nanoparticles and prevent fouling. The calcined biomass-silver nanoparticle (CB-AgNP) composite was characterized using FESEM-EDAX, XRD and TGA. The CB-AgNP composite was used for the first time, as a heterogenous catalyst for reduction of a prominent industrial pollutant, p-nitrophenol. The reduction was carried out in the presence of NaBH₄ in aqueous medium under ambient conditions. Batch experiments were conducted to evaluate the effect of calcination temperature, load of material and its reusability, on the catalytic efficiency of material. It was found that as low as 5 mg mL⁻¹ CB-AgNP material reduced more than 80% and 95% of p-nitrophenol within 1 min and 15 min of exposure, respectively. Rate of PNP reduction was 0.60 mg L⁻¹ min⁻¹. The composite was easily recovered and reused for continuous batches of p-nitrophenol reduction. The efficiency of catalysis decreased with ten cycles of reuse; however, with an intermittent overnight water wash, the material regained its catalytic activity. Furthermore, the CB-AgNP composite also possessed excellent antimicrobial activity against pathogenic microbes. Two strains each of gram + ve and gram − ve bacteria and three strains of pathogenic fungi were used in the antimicrobial studies using well diffusion method and it was found to be active against all the microbes. The CB-AgNP composite is a potential candidate for a reusable heterogenous catalyst for designing continuous flow system for remediation of industrial effluents rich in p-nitrophenol. Its efficacy against common pathogenic bacteria and fungi can be harnessed for simultaneous antimicrobial treatment of the water. Moreover, this antimicrobial property will further inhibit the biofouling and eventual clogging of the material used in a packed column when used for water treatment. [ABSTRACT FROM AUTHOR]