Sulfate-driven enhancement of TiO2 anatase photocatalysis for cationic dye degradation: mechanistic insights and degradation optimization.

The influence of inorganic ions in wastewater matrices on the activity and reuse of TiO2 photocatalysts is an important consideration in photocatalytic water treatment. This study investigates the effect of sulfate ions (SO4 2-) on the photocatalytic degradation of the cationic dye methylene blue (MB) using anatase TiO2 under different irradiation and pH conditions. Photocatalytic performance across repeated treatment cycles was evaluated using cumulative percent degradation (CPD), a parameter introduced in this study, and related to changes in the percent atomic concentration (PAC) of elements detected on the recovered TiO2 surface by X-ray photoelectron spectroscopy (XPS). Under UV irradiation, SO4 2--containing systems exhibited the highest CPD values, reaching 248.10% at pH 4 (TMB4) and 294.23% at pH 10 (TMB8), indicating enhanced cumulative MB degradation in the presence of sulfate under these conditions. Surface characterization revealed changes in the carbon-, nitrogen-, oxygen-, and sulfur-containing species associated with the TiO2 surface following photocatalytic treatment. Carbon-containing surface species increased in the recovered samples and were associated with changes in photocatalytic performance. XPS analysis indicated that sulfur detected on irradiated sulfate-containing samples was attributable to surface-bound sulfate-derived species, with no evidence supporting the formation of discrete molecular sulfur species. Overall, the results demonstrate that SO4 2-, particularly in combination with UV irradiation, can enhance the cumulative photocatalytic degradation of MB while influencing the surface chemistry of anatase TiO2. The findings provide further insight into the relationship between wastewater ionic composition, photocatalytic performance, and surface transformations associated with the repeated application of TiO2 photocatalysts.