Photochemical removal of aniline in aqueous solutions: Switching from photocatalytic degradation to photo-enhanced polymerization recovery

Organic pollutants may be treated by either a degradation process or a recovery process in the view point of sustainable chemistry. Photocatalytic removal of aniline was investigated in aqueous solutions. It was found that the photocatalytic oxidation of aniline resulted in its degradation or polymerization, depending on its concentration. Hence a new treatment strategy was proposed in combination of photocatalytic degradation and polymerization, where the polymerization was in fact a recovery process. When aniline concentration was as low as 0.1 mmol L−1, it was possible to photocatalytically degrade aniline, which could be further enhanced by increasing solution pH, modifying TiO2 surface with the addition of anions, or coupling with the photoreduction of added oxidants. When aniline concentration was increased to about 1 mmol L−1, the photocatalytic oxidation was observed to yield the polymerization of aniline, leading to nanocomposites of polyaniline (PAN) and TiO2. Alternatively, the photo-enhanced chemical polymerization of aniline at higher concentrations (≥50 mmol L−1) in the presence of chemical oxidants produced PAN nanostructures. The conversion of pollutant aniline to valuable PAN nanostructures or nano-PAN/TiO2 composites is suggestive for possible applications in the treatment of aniline wastewaters as a sustainable environmental protection measure.