Influence of pH on the Kinetics and Products of Photocatalytic Degradation of Sulfonamides in Aqueous Solutions

Abstract

The aims of the study were to determine the kinetics of the photocatalytic degradation of six sulfonamides in the presence of TiO₂-P25 in acidic, neutral, and alkaline solutions and to identify the structures of the stable products. It was stated that the pH of the solution significantly affected the photocatalytic degradation rate of sulfonamides in acidic and alkaline environments, and the effect likely depended on the susceptibility of sulfonamides to attack by hydroxyl radicals. In the post-reaction mixture, we identified the compounds resulting from the substitution of the aromatic rings with a hydroxyl group; the amide hydrolysis products; the hydroxylamine-, azo, and nitro derivatives; and the compounds formed via the elimination of the sulfone group. Moreover, previously unknown azo compounds were detected. Some degradation products of sulfonamides may exhibit marked bacteriostatic activity and high phytotoxicity. The azo and nitro compounds formed in an acidic environment may be potentially more toxic to aquatic ecosystems than the initial compounds.

Keywords: degradation products; ecotoxicity; mechanism; photocatalysis; sulfonamides.

Figure 1

Reaction rate constants of the photocatalytic degradation of SNs in acidic, neutral, and alkaline environments.

Figure 2

Relationship between pK_a2 and the degradation rate constant of SNs at pH 10.

Figure 3

Products of the photocatalytic degradation of SAD after 120 min of UV irradiation in the presence of TiO₂-P25. (*) primary amide hydrolysis occurs during QTof analysis.

Figure 4

Products of the photocatalytic degradation of SMX after 120 min of UV irradiation in the presence of TiO₂-P25. (*) primary amide hydrolysis occurs during QTof analysis.

Figure 5

Products of the photocatalytic degradation of STZ after 120 min of UV irradiation in the presence of TiO₂-P25. (*) primary amide hydrolysis occurs during QTof analysis.

Figure 6

Products of the photocatalytic degradation of SFF after 120 min of UV irradiation in the presence of TiO₂-P25. (*) primary amide hydrolysis occurs during QTof analysis.

Figure 7

Products of the photocatalytic degradation of SMR after 120 min of UV irradiation in the presence of TiO₂-P25. (*) primary amide hydrolysis occurs during QTof analysis.

Figure 8

Products of the photocatalytic degradation of SCP after 120 min of UV irradiation in the presence of TiO₂-P25.

Figure 9

Predicted chronic toxicity of SMX, of its photocatalytic degradation products (compounds marked with letters A through I in Figure 4), and of maleic (M…), fumaric (Fu…), acetic (Ac…), oxylatic (Ox…), and formic (Fo…) acids to selected aquatic organisms using the ECOSAR model.

Figure 10

Chromatogram of SMX solution after 120 min of irradiation in the presence of TiO₂ P25 at pH 3.