Photodegradation of salicylic acid in aquatic environment: Effect of different forms of nitrogen

Salicylic acid (SA), as an extensively used compound, can be detected in a great variety of environmental water samples. Photodegradation is important in many ways. The present study concerns the environmental behavior of SA under simulated sunlight. A kinetic model was used for SA degradation in water, and the variations of the photodegradation of SA in the presence of different initial concentrations, different oxygen levels, different forms of nitrogen and different pE values in the aquatic environment were determined. Experiments demonstrated that the photodegradation process had pseudo-first-order reaction kinetics. The photodegradation rate decreased with increasing initial concentration and increased with increasing oxygen level. The NO3− and NO2− ions promoted photodegradation of SA, but increases of NH4+ concentration had no effect. The form of nitrogen depends on pE, which therefore has a significant influence on the photodegradation of SA. When the pE value increased gradually, there was a transformation of NH4+ to NO2− and then to NO3−. The photodegradation rate of SA first increased, then decreased and finally increased again. When NO2− and NH4+ coexisted, the photodegradation rate was almost the same as it was in the presence of NO2− alone. When NO2− and NO3− coexisted, the promoting effect on the photodegradation SA was less than the sum of the partial promoting effects. The results indicated that NO2− had an obvious antagonistic action on NO3− when NO3− and NO2− coexisted in the aquatic environment.