Comparative study of visible-light-driven photocatalytic inactivation of two different wastewater bacteria by natural sphalerite

The photocatalytic inactivation of two different wastewater bacteria, Gram-positive (+ve) bacterium Microbacterium barkeri and Gram-negative (-ve) bacterium Escherichia coli, were comparably investigated with natural sphalerite (NS) as a novel low-cost photocatalyst. The natural sphalerite was able to inactivate 10⁵cfu/mL of the Gram-positive (+ve) bacterium M. barkeri within 10h at neutral pH while 10⁷cfu/mL of the Gram-negative (-ve) bacterium E. coli was inactivated within 6h. In addition, the pH effect on the photocatalytic inactivation of bacteria was studied, and the results suggested that the amount of H₂O₂ produced under different pH influenced the bacterial inactivation efficiency. The major reactive species for photocatalytic inactivation by the NS were determined using multiple scavengers, and H₂O₂ was found to be the major ROS in M. barkeri inactivation, while both H₂O₂ and e^- contributed to E. coli inactivation. Different mechanism between Gram-positive (+ve) and Gram-negative (-ve) bacteria was mainly due to the different thickness of the cell wall, as indicated by direct observation of the cell wall and cell membrane by transmission electron microscopy and leakage detection of potassium ions.