Extracellular synthesis of silica particles was carried out by Fusarium oxysporum mycelia using rice husk ash (RHA) as raw material. RHA before and after bioprocessing was analyzed by SEM and specific surface area. Dissolved silica was quantified using colorimetry. Incubation of rice husk ash with F. Oxysporum at 28°C and pH 6.8 indicated that 80% silica was dissolved and micrographs of RHA before and after reaction with fungi showed that the morphology of silica particles changed and the average size decreased (~600 to ~5 µm). Moreover, specific volume pore of particles was reduced from 0.026 to 0.013 cm3/g and surface area increased from 115 to 125 m2/g. Dissolved silica corresponds to 177 m2/g. It is concluded that synthesis of oxide materials using fungal biotransformation is a viable process to obtain added value products from agro-industrial waste materials.

Se evaluaron cinco concentraciones de triptona (0.2%, 0.4%, 0.6%, 0.8% y 1.0%) en procesos de biomineralización de carbonato de calcio, utilizando acetato de calcio al 0.5%. Todos los ensayos se hicieron con una cepa de Bacillus cereus, aislada de los jardines de la Universidad Pontificia Bolivariana (Medellín, Antioquia, Colombia). Los experimentos fueron monitoreados con mediciones de pH y análisis mineralógicos de difracción de rayos X (XRD), espectroscopía de infrarrojo con transformada de Fourier (FTIR) y microscopía electrónica de barrido (SEM). La vaterita fue el polimorfo predominante en los precipitados después de 6 días de proceso. Sin embargo, una proporción considerable de calcita (por encima de 30%) apareció cuando la concentración de triptona fue de 0.4% y menor. Adicionalmente, los experimentos tuvieron una formación de precipitados similar (entre 3.2 y 3.6 g/L), menos el ensayo con 0.2% de triptona, que presentó una baja producción de carbonato de calcio (1.79 g/L), indicando que una concentración de triptona menor a 0.4% limitaría el metabolismo bacteriano y la formación de CO2, necesaria para la producción de carbonato de calcio.

The biotransformation of rice husk ash (RHA) by Fusarium oxysporum to generate silica nanoparticles was carried out using two different commercial growth media: malt-glucose (MG) and malt-glucose-yeast-peptone (MGYP). Biomass production, substrate consumption, organic acids production, and solubilized silica were measured during RHA biotransformation. Extracellular proteins were analyzed by SD-PAGE. Silica nanoparticles were analyzed by XRD, zeta potential, SEM, and TEM. The results showed that the production of organic acids was not directly related to the solubilization of silica. Solubilization and stabilization of silica occur mainly in the exponential growth phase of F. oxysporum, which are associated with the action of extracellular proteins with sizes 24, 55, and 70 kDa. MG medium presented the best performance for the growth of F. oxysporum and production of semicrystalline, quasi-spherical silica nanoparticles in the range of 2−8 nm.