Photocatalytic Degradation of p ‐Nitrophenol in an Annular Column Photoreactor and the Intermediates

  The photocatalytic degradation of p ‐nitrophenol using a titanium dioxide suspension was studied in an annular column photoreactor operating in batch recycle mode with an aerated reservoir tank. The dependency of the process efficiency on the initial PNP concentration was quantitatively defined using an exponential function. The degradation rate was highest at pH 7‐7.2. The appearance of p ‐benzoquinone, p ‐hydroquinone, and phenol during the degradation process was confirmed by high‐performance liquid chromatography analysis. The formation of similar intermediates during the microbial degradation of PNP has been reported previously. The formation of hydroxyl radicals is predominant in the PNP photodegradation route, and fluctuations of the chemical oxygen demand may be indicative of the appearance of unidentified and probably nonbiodegradable intermediates formed during photocatalysis. These compounds likely contribute to the COD variations. Herein, the results of PNP removal via photocatalytic degradative reactions are discussed, and the intermediates are compared to those observed in enzymatic reactions.