Formation Of Nitrophenolic Byproducts In Sulfate Radical Oxidation Processes

Sulfate radical(SO₄^·-)-based advanced oxidation processes are considered as promising technologies for water treatment and in situ remediation of contaminated soil and/or groundwater.However,little is known concerning the potential issues and limits of this technology.In this study,multiple spectroscopic approaches including Fourier-transform infrared spectrometry,¹⁵N labeling in combination with mass spectrometry and nuclear magnetic resonance were applied to trace the transformation of inorganic nitrogen,i.e.,nitrite（NO₂^-）and ammonium（NH₄⁺）,in the heat activated peroxydisulfate（PDS）oxidation process in both water and soil environments.The present study demonstrated that both NO₂^-and NH₄⁺could be transformed to nitrophenolic byproducts including 4-nitrophenol,5-nitrosalicylic acid,2,4-dinitrophenol,3,5-dinitrosalicylic acid,etc.The toxicity change of reaction mixtures was in response to the formation of nitrophenolic byproducts.SO₄^·-was presumed to play a key role in this process.It can react with NO₂^-and NH₄⁺to form nitrogen dioxide radicals（NO₂^·）.Moreover,it reacts with natural organic matter（NOM）to form phenoxy radicals to which NO₂^·can be incorporated immediately,generating nitrophenolic byproducts.Two types of nitration sites in NOM molecules can be transformed to phenoxy radicals upon SO₄^·-oxidation.Fast sites associated with the phenolic functionality in NOM were oxidized to phenoxy radicals directly,while slow NOM sites associated with the aromatic carboxylic functionality underwent the decarboxylation-hydroxylation reaction to in situ generate phenolic intermediates,which were further transformed to phenoxy radicals.This study further revealed that NO₂^-was rapidly oxidized by SO₄^·-,generating NO₂^·directly which reacted readily with the fast phenolic sites.While NH₄⁺was observed to react with SO₄^·-generating NH₂^·first and then form NO₂^·through a radical chain reaction with the assistance of oxygen molecule,which engaged the slow sites associated with the aromatic carboxylic functionality.When NO₂^-and NH₄⁺were present together,both fast and slow sites were utilized,and these two inorganic nitrogen species acted synergistically on the formation of nitrophenolic byproducts.In the earlier phase,NO₂^-did the main contribution to the nitrophenolic byproducts formation.With prolonging the reaction time,the contribution of NH₄⁺became increasingly important and it outweighed that of NO₂^-.Experimental data showed that the formation of total organic nitrogen for a 48 h contact time reached 5.07μM with 35.6%originated from NO₂^-and 64.4%from NH₄⁺,during SO₄^·-oxidation of NOM（4.96 mg/L organic carbon）in the presence of both NO₂^-（0.1 m M）and NH₄⁺（1.0 m M）,while the sum of organic nitrogen formed in the presence of each alone was only 1.51μM.In addition,the presence of NH₄⁺could inhibit the further degradation of the already-formed mono-nitrophenolic byproducts from NO₂^-,and instead,transformed them to di-nitrated ones.Overall,NH₄⁺could have a more significant contribution to the formation of nitrophenolic byproducts.Because of the omnipresence of NO₂^-and NH₄⁺in the environment,formation of nitrophenolic byproducts in SO₄^·-oxidation processes might be ubiquitous,which should be taken into consideration in the feasibility evaluation of applications of this technology for environmental remediation.