Photodegradation of nitrite in lake waters: role of dissolved organic matter

Here we studied the degradation rate of nitrite (NO2-), added to lake water at sub-micromolar levels, upon ultraviolet (UV) irradiation. NO2- photodegradation was considerably faster in lake water compared with ultra-pure water. A key issue was the presence in lake water of hydroxyl radical ((OH)-O-center dot) scavengers that inhibited the reaction between NO2- and (OH)-O-center dot. Such a reaction, while causing additional NO2- transformation, produced nitrogen dioxide (NO2 center dot) that was subsequently involved into the regeneration of NO2- by dimerisation or the reaction with nitric oxide (NO center dot). The scavenging of (OH)-O-center dot by compounds different from NO2- (mainly dissolved organic matter, DOM) prevented the regeneration reactions from taking place, and enhanced the phototransformation of NO2-. Model calculations for the direct photolysis of NO2-, applied to the lake water samples, yielded a NO2- half-life time of around three weeks in the mixing layer of the lakes because of photodegradation. Therefore, we conclude that photodegradation is a potentially important process to control the concentration of NO2- in shallow lakes, or in deeper ones under stratification conditions.