Green nanocatalyst for the photocatalytic degradation of organic pollutants in petroleum refinery wastewater: Synthesis, characterization, and optimization

The synthesis of green nanocatalysts is an emerging branch of current nanotechnology due to its numerous advantages, including high efficiency, sustainability, cost-effectiveness, biocompatibility, and eco-friendliness. In this study, a thermal technique was employed to prepare a novel green nanocatalyst (GNC) from the sepals of waste tomato (WT) plants, and this GNC was investigated to determine its efficiency in degrading the chemical oxygen demand (COD) during a photocatalytic process of simulated petroleum refinery wastewater (PRW). The characterization of the waste tomato-green nanocatalyst (WT-GNC) was performed using various techniques, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray (EDX), Brunauer-Emmett-Teller (BET) analysis, and Fourier-transform infrared (FTIR) spectroscopy, which revealed that the synthesized WT-GNC possessed an excellent catalytic quality. This work examined the key factors affecting the degradation rates, such as the type of light source (UV or LED) and its arrangement (outside or inside the reactor), aeration rate, pH, catalyst dose, irradiation time, temperature, and initial COD concentration. The WT-GNC degraded 99.9 % of the COD (residual = 1 ppm) under the optimal operating conditions (pH = 7, WT-GNC dose = 0.25 g/L, irradiation time = 90 min, temperature = 25 degrees C, and initial COD = 1326 ppm) under UV lamps located around the inside of the reactor wall. Ultraviolet (UV) light is highly effective in degrading the COD compared to visible LED lights. The reaction followed the pseudo-first-order kinetic model (R-2 = 0.998) with k = 0.0367 min(-1). The WT-GNC exhibited good reusability and stability and could reduce 76.3 % of the COD after the fifth consecutive cycle. Therefore, this novel GNC can serve as a cost-effective, efficient, stable, and eco-friendly photocatalyst to lower the COD of PRW to meet the World Health Organization (WHO) standards. In addition, it can be recovered and reused several times.