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

Here we studied the degradation rate of nitrite (NO-2), added to lake water at sub-micromolar levels, upon ultraviolet (UV) irradiation. NO-2 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) scavengers that inhibited the reaction between NO-2 and OH. Such a reaction, while causing additional NO-2 transformation, produced nitrogen dioxide (NO-2) that was subsequently involved into the regeneration of NO-2 by dimerisation or the reaction with nitric oxide (NO). The scavenging of OH by compounds different from NO-2 (mainly dissolved organic matter, DOM) prevented the regeneration reactions from taking place, and enhanced the phototransformation of NO-2. Model calculations for the direct photolysis of NO-2, applied to the lake water samples, yielded a NO-2 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 NO-2 in shallow lakes, or in deeper ones under stratification conditions.