Differentiation of Pathways of Nitrated Byproduct Formation from Ammonium and Nitrite During Sulfate Radical Oxidation

Recent studies found that both nitrite (NO₂^-) and ammonium (NH₄⁺) lead to nitrophenolic byproducts in SO₄^•- oxidation processes, during which NO₂^• generated through the oxidation of the inorganic nitrogen by SO₄^•- is the key nitrating agent. This study demonstrates that the formation of phenoxy radicals to which NO₂^• can be incorporated immediately is another governing factor. Two types of sites having distinct reactivities in natural organic matter (NOM) molecules can be transformed to phenoxy radicals upon SO₄^•- oxidation. Fast sites associated with phenolic functionalities are primarily targeted in the reaction sequence involving NO₂^-, because both are preferentially oxidized. Following the depletion of NO₂^-, NH₄⁺ becomes the main precursor of NO₂^• that interacts with slow sites associated with the carboxylic functionalities. Experimental data show that the formation of total organic nitrogen in 24 h reached 6.28 μM during SO₄^•- oxidation of NOM (4.96 mg/L organic carbon) in the presence of both NO₂^- (0.1 mM) and NH₄⁺ (1.0 mM), while the sum of those formed in the presence of each alone was only 3.52 μM. Results of this study provide further insights into the mechanisms of nitrated byproduct formation when SO₄^•- is applied for environmental remediation.