Background: Photo-degradation of organic contaminants into non-hazardous mineral compounds is emerging as
a strategy to purify water and environment. Tremendous research is being done using direct solar light for these
purposes. In this paper we report on optimum conditions for complete mineralization of aqueous methyl orange
using lab-prepared ZnO nanopowder catalyst under simulated solar light.
Results: Nano-scale ZnO powder was prepared in the lab by standard methods, and then characterized using electronic
absorption spectra, photolumenscence emission (PL) spectra, XRD, and SEM. The powder involved a wurtzite structure
with ~19 nm particles living in agglomerates. Photo-degradation progressed faster under neutral or slightly acidic
conditions which resemble natural waters. Increasing catalyst concentration increased photodegradation rate to a certain
limit. Values of catalyst turn over number and degradation percentage increased under higher light intensity, whereas the
quantum yield values decreased. The photocatalytic efficiency of nano-ZnO powders in methyl orange photodegradation
in water with solar light has been affected by changing the working conditions. More importantly, the process may be
used under natural water conditions with pH normally less than 7, with no need to use high concentrations of catalyst or
contaminant. The results also highlight the negative impact of possible high concentrations of CO2 on water purification
processes. Effects of other added gaseous flows to the reaction mixture are also discussed.
Conclusion: ZnO nano-particles are useful catalyst for complete mineralization of organic contaminants in water.
Photo-degradation of organic contaminants with ZnO nano-particles, methyl orange being an example, should
be considered for future large scale water purification processes under natural conditions