Real-Time Adsorption and Photodegradation Investigation of Dye Removal on g-C3N4 Surface by Attenuated Total Reflectance Induced Evanescent Spectroscopy

In this work, the adsorption and photodegradation performance of dye pollutant methylene blue (MB) on the g-C3N4 surface was real-time investigated based on attenuated total reflectance induced evanescent spectroscopy (ATR-ES). By coating g-C3N4 on a silica optical fiber (SOF) surface coupled to LED, the produced ATR-ES was interacted with MB adsorbed on the g-C3N4 interface. Therefore, the measurement processes were in situ monitored and real-time evaluated according to ATR-ES change. For adsorption investigation with a red-LED, the change of “red” ATR-ES indicated the adsorption process was spontaneous. At the first stage, the fast adsorption was mainly due to electronic attraction. At the second stage, the slow adsorption was mainly due to the π–π interaction. For photodegradation investigation with a UV-LED, the change of “violet” ATR-ES indicated MB was oxidized by a hydroxyl radical (•OH) and superoxide radical (•O2–) to upset adsorption–desorption equilibrium. The surface apparent adsorption rate was 16 times as high as the apparent photodegradation rate constant, which indicated that the photodegradation process was the main rate control step of photocatalytic degradation reaction. The performances were characterized and verified by SEM measurements, zeta potential analysis, XRD analysis, FTIR investigation, and fluorescence imaging. By utilizing ATR-ES properties on the SOF surface, the adsorption-degradation interaction was continuously real-time monitored.