A reusable visible driven N and C–N doped TiO2 magnetic nanocomposites for photodegradation of direct red 16 azo dye in water and wastewater

Abstract

ABSTRACT The visible active N-doped TiO2/ZnFe2O4 (urea–TiO2/ZnFe2O4) and CN-codoped TiO2/ZnFe2O4 (L-asparagine–TiO2/ZnFe2O4) nanocomposites were successfully synthesized by the sol–gel–hydrothermal method for direct red 16 (DR16) photodegradation. Their properties of the prepared nanocomposites were analysed using XRD, FT-IR, FE-SEM, EDX, DRS and PL tests. The DRS and PL results confirmed a narrow band-gap energy and low recombination rate of photo-produced electron and hole pairs, respectively. The effect of adding various dopant agents (urea and L-asparagine) with different loadings and magnetic nanoparticle (ZnFe2O4) into TiO2 sol on the photodegradation of DR16 was also evaluated. As a result, the L-asparagine (2 wt. %)–TiO2/ZnFe2O4 is the best photocatalyst compared to the other modified TiO2 nanocomposites due to its narrow band gap and high quantum efficiency. The catalyst concentration (1–2 g/L), DR16 concentration (25–45 ppm), initial pH (4–10), and irradiation time (30–90 min) as numerical variables were also considered for photocatalytic process analysis and moulding by central composite design (CCD). The increase in the pH and dye concentration reduces the photodegradation efficiency while irradiation time and catalyst concentration effectively improved its photodegradation efficiency. The DR16 was completely removed at 25 ppm of DR16, initial pH of 4 and 1.5 g/L of photocatalyst after 90-min irradiation. The photoactivity test was also repeated four times by reused L-asparagine–TiO2/ZnFe2O4 photocatalyst at optimum conditions. The decrease of dye degradation and loss of photocatalyst were not significant which was approved by the good performance and high recovery capability of the prepared nanocomposite.