Spontaneous oxidative degradation of indigo carmine by thin films of birnessite electrodeposited onto SnO2

This paper describes the successful use of thin films of birnessite to remediate aqueous solutions containing indigo carmine (IC) dye. This latter is one of the oldest and most important dyes used and released in the effluents of many industries, such as textile, paper and plastics. Moreover, this anionic dye belongs to a highly toxic class of dyes (indigoïd). We show that thin films of birnessite, electrodeposited onto a cheap transparent semiconductor substrate (SnO2), can totally discolor the IC solutions at room temperature after only 1 h in the best conditions. The mass spectrometry analyses reveal the presence of intermediate reaction products (m/z = 226 amu, m/z = 244 amu and m/z = 216 amu) during interaction confirming the real degradation of dye. Moreover, nitrate and ammonium ions are quantified showing that thin films of birnessite can degrade totally (mineralise) and spontaneously more than 60% of indigo carmine into these mineral species after 24 h in the best conditions. These results confirm that birnessite thin films appear as a very interesting material for the development of a simple and ecological method of wastewater remediation.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Study of spontaneous reactivity of birnessite towards indigo carmine, a toxic dye. ► Rapid degradation and mineralisation without energy supply. ► Importance of agitation on the degree of mineralisation. ► Highlighting the good spontaneous reactivity of a non-toxic material. ► Interest of using thin layer for understanding mechanism at solid–liquid interfaces.