DV photolysis of nitrate: Effects of natural organic matter and dissolved inorganic carbon and implications for UV water disinfection

Nitrite (NO2-) formation during ultraviolet (UV) photolysis of nitrate was studied as a function of pH and natural organic matter (NOM) concentration to determine water-quality effects on quantum yields and overall formation potential during UV disinfection of drinking water with polychromatic, medi um-pressure (MP) Hg lamps. Quantum yields measured at 228 nm are approximat ely 2 times higher than at 254 nm under all conditions studied. In the abse nce of NOM, NO2- quantum yields decrease with time. With addition of NOM, i nitial quantum yields increase, and the time-dependent decrease is eliminat ed. At 15 ppm dissolved organic carbon (DOC) as NOM, the quantum yield incr eases with time. Dissolved inorganic carbon significantly decreases NO2- yi elds at pH 8 but not pH 6, presumably by reaction of CO2(aq) with peroxynit rite, a major intermediate in NO2- formation. The results indicate importan t and previously unrecognized roles for NOM and CO2(aq) in nitrate photolys is. When photolysis was carried out using the full spectrum MPUV lamp and g ermicidally relevant UV doses, NO2- concentrations remained well below the U.S. maximum contaminant level of 1 ppm N, even with nitrate initially pres ent at 10 ppm N. Under current U.S. regulations, NO2- formation should not pose a significant problem for water utilities during UV disinfection of dr inking water with MP Hg lamps.