Removal and regrowth inhibition of microalgae using visible light photocatalysis with ZnO nanorods: A green technology

Algal biofouling can be a major problem during membrane filtration processes reducing membrane efficiency. Removal of microalgae by visible light photocatalysis using zinc oxide (ZnO) nanorods was studied in this work. ZnO nanorods were grown on polypropylene support substrates. The treatment unit was constructed by incorporating ZnO nanocoated substrates in a glass tube. Anti-algal activity of the treatment units were tested using green microalga, Dunaliella salina, of 10⁷ cells/mL concentration, which is higher than the concentration of cells during algal blooms. Nearly total algal cell inactivation was achieved within 2 h of continuous visible light illumination in the presence of nanocoated support substrates, as determined by flow cytometry analysis (98%) and trypan blue staining (95%). Uncoated support substrate under light illumination did not lead to algal cell mortality (1.7%). Complete inhibition of any regrowth of algal cells treated with nanocoated substrates was confirmed as no significant changes in the total number of cells were observed even after 2 weeks of incubation of the treated culture. The anti-algal activity of ZnO nanorods was attributed to the formation of reactive oxygen species (ROS) through photocatalytic processes. ZnO nanorod coated substrates used in the treatment units could be a suitable green method to control membrane fouling in water treatment plants avoiding the utilisation of harmful chemicals.