Time-dependent mechanistic insight into photo-degradation of mixed hydrophobic disperse dyes by magnetically separable nitrogen iron codoped titania under visible light using process variable optimization

Abstract

This is the first report of the time-dependent mechanistic insight into photo-degradation of a mixture of Navy Blue 3G, Scarlet RR, Scarlet 3R dyes using visible light active and magnetically separable ternary γ-Fe2O3/N,Fe–TiO2 prepared by the ultrasonic-hydrothermal method. In the present work, samples collected on an hourly basis over a period of 4 h to propose the time-dependent mechanistic pathway and formation of transient intermediates delineated by LC-MS analysis. The decrease in a number of transient intermediates formed with respect to time of degradation supported the effective mineralisation process of disperse dyes, known to be difficult to degrade due to their hydrophobic nature. After 4 h, only one major intermediate was obtained from Navy Blue 3G along with two minor intermediates from Scarlet RR were observed while Scarlet 3R underwent complete mineralisation. The optimization of process parameters was done by response surface methodology (RSM) based on central composite design (CCD). The degradation was monitored by UV and chemical oxygen demand (COD) analysis which indicated complete decolouration and degradation up to 78% under the optimum conditions of initial concentration 40 mg/L, pH 7.5 and with a catalyst loading of 1.0 g/L using 1.5 ml of H2O2 within 4 h as obtained by RSM. The enhanced activity of γ-Fe2O3/N,Fe–TiO2 can be ascribed to a considerable shift in bandgap, enhanced surface area and generation of reactive species.