Role of hydrogen peroxide and hydroxyl radical in producing the residual effect of ultraviolet radiation.

Residual effect of UV radiation on Microcystis aeruginosa and Escherichia coli K12 A/lambda (F+) was studied under various conditions to examine if the formation of hydrogen peroxide (H2O2) and hydroxyl radical (OH.) can explain the residual biocidal action of UV radiation. Survival of test organisms, that is, M. aeruginosa and E. coli K12 A/lambda (F+), in UV-irradiated synthetic water was monitored to assess the residual effect after the UV radiation. Synthetic water with various compositions was used to understand the role of transition metals and photosensitizing organic molecules in producing the residual biocidal action. Ultraviolet-irradiated synthetic water containing a photosensitizing element and ferric ion (Fe3+) showed residual biocidal effect for more than 7 days. No residual effect was observed in the absence of any photosensitizing element. A close relationship was observed between the residual effect and the formation of hydrogen peroxide in the UV-irradiated water. Scavenging of hydrogen peroxide and hydroxyl radical from the UV-irradiated water significantly reduced the residual biocidal action. Ultraviolet-irradiation of synthetic water containing ferric ion was found to promote the formation of ferrous ion (Fe2+). Because ferrous ion can produce reactive species in the presence of hydrogen peroxide, transition metals and hydrogen peroxide are believed to play a key role in producing the residual effect of UV radiation through the formation of reactive species, for example, hydroxyl radical.