Nigella sativa seed based nanocomposite-MnO2/BC: An antibacterial material for photocatalytic degradation, and adsorptive removal of Methylene blue from water.

Antimicrobial Nigella sativa seed-based nanocomposite, MnO₂/BC, was synthesized and utilized for the water purification through adsorption, and the photocatalytic degradation. MnO₂/BC was prepared by co-precipitation method, and characterized using FT-IR, XRD, SEM, TEM, TGA, and DSC techniques. The composite was investigated for inhibition of bacterial cells growth. FT-IR spectrum indicated the presence of oxygenous groups on the surface; TGA and DSC showed thermal degradation; and XRD, SEM, and TEM investigations indicated amorphous, and porous nature of MnO₂/BC having particle size of 190-220 nm. The nanocomposite inhibited the growth of both Gram-positive and Gram-negative bacteria cells in water. The adsorption of Methylene blue from water was investigated in batch method in terms of amount of MnO₂/BC, dye concentration, pH, time, and temperature. 1.0 g L^-1 of MnO₂/BC removed more than 98% of Methylene blue from aqueous solution having concentration of 10 mg L^-1 and pH 7.0 at 27 °C. The maximum Langmuir adsorption capacity of MnO₂/BC was 185.185 mg g^-1 at 45 °C. The adsorption was an endothermic process which obeyed Freundlich isotherm, and pseudo-second order kinetics. Therefore, the Methylene blue binding onto MnO₂/BC surface was site-specific partially through the weak hydrogen bonding, and electrostatic interactions. The photocatalytic activity of MnO₂/BC has been investigated by degrading the Methylene blue molecules/ions in water under the sunlight and 85% of degradation was achieved during 120 min irradiation. The dye was desorbed at lower pH and regenerated MnO₂/BC was used for second cycle of Methylene blue adsorption. The results obtained for this study are much better than the previous Methylene blue adsorption studies with acid washed Black cumin seeds and MnFe₂O₄/BC for which the capacities were 73.529 mg g^-1 and 10.070 mg g^-1 at 27 °C, respectively (J. Mol. liq. 2018a, 264, 275-284; J. Clean. Prod. 2018a, 200, 996-1008).