Mechanistic of photocatalytic decolorization and mineralization of methyl orange dye by immobilized TiO2/chitosan-montmorillonite

•CS-MT film adhered stronger and swelled less than CS film.•Immobilized TiO2/CS-MT removed MO dye faster than TiO2.•The MO dye degradation occurred via the e−/O2- route.•Two photocatalytic mineralization pathways were proposed. The mechanistic of photocatalytic decolorization and mineralization of methyl orange (MO) dye have been successfully studied using immobilized titanium dioxide/chitosan-montmorillonite (TiO2/CS-MT), a combination of TiO2 as the top layer and CS-MT as the sub-layer on a glass plate. The immobilized CS-MT film was selected over the CS film since the former adhered stronger and swelled less than the latter which showed its favorability in the aqueous medium. This bilayer photocatalyst operated via a simultaneous occurrence of adsorption of MO dye from the CS-MT layer and photocatalytic degradation by TiO2 layer. The bilayer photocatalyst could remove the MO dye from the solution 3 times faster than the single TiO2 within 90 min of irradiation under a UV–vis lamp due to strong adsorption of MO by the CS-MT sub-layer. The mechanistic study revealed that the degradation occurred via the e−/O2- route while the LC–MS analysis suggested the possible degradation pathway of MO dye. It was found that the eluent used for separation of compounds in the LC–MS analysis influenced the intermediates detected for which two degradation mechanisms were proposed.