Abstract
The application of photocatalysis for the disinfection of water has been extensively reported over the past 30 years. Titanium dioxide (TiO2) has been the most widely and successfully used photocatalyst to date; however, it is not without its limitations. Frequently observed long lag times, sometimes up to 60 min, before bacterial inactivation begins and the presence of residual microorganisms, for example, up to 104 colony forming units, remaining after treatment are ongoing challenges with this particular photocatalyst. It is therefore important to find alternative photocatalysts that can address these issues. In this study, we compared the disinfection capacity of TiO2 with that of zinc oxide (ZnO) using Escherichia coli as a model organism in both a suspended and immobilized catalyst system. Our results showed that ZnO was superior to TiO2 in a number of areas. Not only were bacterial rates of destruction much quicker with ZnO, but no lag time was observed prior to inactivation in suspended systems. Furthermore, complete bacterial destruction was observed within the treatment times under investigation. The greater efficiency of ZnO is believed to be due to the decomposition of the bacterial cell wall being driven by hydrogen peroxide as opposed to hydroxyl radicals. The results reported in this paper show that ZnO is a more efficient and cost-effective photocatalyst than TiO2 and that it represents a viable alternative photocatalyst for water disinfection processes.