Enhanced exopolysaccharide production in gamma irradiated Bacillus subtilis: A biofilm-mediated strategy for ZnO nanoparticles removal

Biofilm-mediated strategy was studied for ZnO nanoparticle removal from aqueous media. Bacillus subtilis isolated from the soil rhizosphere was selected based on its high viscosity (133 Pa/s) of the cultivated culture and biofilm formation. The bacterium was exposed to gamma-irradiation to enhance EPS production along with its cultivation in EPS-producing media. The results show an increase in viscosity that reached 160 Pa/s at 2 kGy. EPS production increased from 4.45 to 7.95 mg/mL and the protein/carbohydrate ratio increased from 3 to 4.4 which reflects the stickiness of EPS. Thermal Gravimetric Analysis (TGA) showed 2 phase weight loss for gamma irradiated EPS and defined protein peaks when characterized using Matrix Assisted Laser Desorption Ionization-Time of Flight (MALDI-TOF). Native and gamma-irradiated Bacillus subtilis cells with their enhanced EPS were grown as a biofilm on sterile waste gauze fabric, Scanning Electron Microscopy (SEM) showed an increased biofilm attachment in gamma-irradiated samples. The latter was used for the removal of ZnO NP from aqueous media. Energy dispersive X-ray (EDX) mapping confirms that ZnO NPs were entrapped within the carbon and oxygen elements forming the biofilm with net intensities of 14.04, 1713, and 1190, respectively. The results confirm that biofilm-mediated strategy is effective in nanoparticles removal.