Green-synthesized ZnO via Hyphaene thebaica fruit extracts: Structure & catalytic effect on the ozonation of Coralene Rubine-S2G azo disperse dye

ZnO nanoparticles were successfully synthesized using aqueous extracts of Doum palm tree ( Hyphaene thebaica ) fruits. The green-synthesized ZnO particles annealed at 400, 500 and 600 °C, alongside commercial ZnO, were structurally and optically characterized using SEM, TEM, XRD, FTIR, TGA, BET surface area analysis and Diffuse reflectance spectroscopy. SEM and TEM showed commercial ZnO (ZnO-Comm) to consist largely of nanorods, while green-synthesized ZnO particles consisted essentially of semi-spherical particles with assorted shapes. XRD analysis showed the formation of crystalline ZnO in the hexagonal phase with average crystallite size varying in the order ZnO-500 (31.32 nm) ≈ ZnO-Comm (31.32 nm) < ZnO-600 (43.46 nm) < ZnO-400 (44.59 nm). BET surface area analysis showed powders to be largely macroporous. Compared to commercial ZnO, sub-stoichiometric green-synthesized ZnO nanoparticles (band gap 3.14–3.27 eV) were shown to have a catalytic effect on the ozonation kinetics of Coralene Rubine S-2G, reducing the overall reaction time by 20 mins. Based on the Langmuir-Hinshelwood model, the ozonation process exhibited a 2-stage rate kinetics profile with 2nd stage rate kinetics being 3–8 times faster than the first stage (ZnO-400 (0.04382 min −1 ); ZnO-500 (0.0566 min −1 ), ZnO-600 (0.04176 min −1 )). Instead of ZnO dissolution accounting for the exponential increase in 2nd stage kinetics we submit that a sharp drop in Coralene Rubine-S2G concentration with respect to OH radicals, as ozonation nears termination, is what accounts for a faster 2 nd stage rate kinetics. For the green-synthesized ZnO, rate kinetics of the ZnO-catalyzed ozonation of CR-S2G varied proportionally to changes in surface area and inversely to changes in the grain size. This work shows that due to the presence of oxygen vacancies and phytochemically-derived residual organics, green-synthesized ZnO nanoparticles can be a better and eco-friendly alternative to commercially produced ZnO for catalytic ozonation of the azo disperse dye, Coralene Rubine-S2G, reported here for the first time.