Carbon dot-chitosan nanocrystal photocatalyst for the photodegradation of methyl violet and removal of degradation products for the pretreatment of textile industry effluent.

A photocatalyst was obtained by synthesising boron and nitrogen co-doped carbon dots directly onto the surface of chitosan nanocrystals, which stabilised both components, improving the intrinsic photocatalytic properties of carbon dots as well as the sorption properties of chitosan nanocrystals. The material was characterised using FTIR, XRD, SEM, TEM, EDX, BET, BJH, and spectrofluorimetry. The electrical properties were characterised using electrochemical and Mott-Schottky impedance spectroscopy; the hybrid had a band gap of 3.55 eV and the valence and conduction bands had potentials of approximately +2.7 and -0.84 V, respectively. The photocatalyst degraded a 100 mg/L methyl violet dye solution within 54 h under ultraviolet irradiation at a catalyst loading of 400 mg/L and at neutral pH at 98% efficiency to produce benzoic acid and Michler's ketone intermediates. The catalyst had an adsorption percentage of 10% and a light-driven photocatalytic rate of 15%; however, this was attributed to the high initial concentration of target molecules, which initially prevented adequate light penetration. The reaction followed pseudo-first-order kinetics and had a rate constant of 0.00179 min-1. Total organic content studies showed that the catalyst removed 89.2% of organic carbon. Isotherm studies showed that the adsorption of Michler's ketone was favourable (1/n = 0.03), adsorption occurs on a homogenous surface (n = 1.15), and the material had a maximum adsorption capacity of 412 mg/g. The photocatalyst was reusable for up to three 54-h cycles without regeneration; however, regeneration with 0.1 M sodium hydroxide restored the catalyst to its original efficiency.