Chemically Enhanced Sunlight for Killing Bacteria

Solar ultraviolet (UV) photocatalyzed oxidation of chemicals with titanium dioxide (TiO₂) has received considerable attention. Much less recognized, however, is the ability of the same system to destroy bacteria. This study examined this phenomenon and the conditions that effect it. Bacteria in aqueous solution were given solar exposure with titanium dioxide and their survival with time was detected. Lamps with a predominantly solar ultraviolet spectrum were also used in the experiments. Without exposure to UV light, TiO₂ does not affect the bacteria. However, several common bacteria were killed in just a few minutes on solar exposure in the presence of TiO₂. Whereas without TiO₂ it took more than an hour to destroy them. A concentration of 0.01 percent TiO₂ was most effective in killing bacteria and tenfold concentrations lower or higher were successively less effective. Inorganic and organic compounds in solution, even in small amounts, interfered with the efficiency of killing. An alkaline solution also reduced the bactericidal activity. Circulation and agitation provided by stirring to keep the TiO₂ particles suspended reduced the time necessary to kill the bacteria. Time-intensity curves for killing bacteria were the same general shape with or without TiO₂. This suggests that TiO₂ served merely as a catalyst to increase the rate of the reaction but that the mechanism of action was not changed. The shape of the curves shows that the organisms are sensitized with a minimum intensity of radiation and that an increase doesn’t greatly increase the rate of the kill. Below this critical intensity, however, the time required for killing markedly increases as the intensity is decreased.