Chemical oxygen demand reduction

Chemical oxygen demand (COD) of organic content (phenol) in water samples was observed for every trial to investigate the influence of changing various operational parameters on the removal of COD. The COD experiment included oxidizing the liquid samples’ organic content under acidic conditions at 148 °C for 2 h. Strong oxidizing agents of potassium dichromate and sulfuric acid oxidized the organic matter in the presence of silver sulfate as a catalyst. These compounds are all present in the sample cuvettes LCI400 (HACH LANGE LTD, Manchester, UK) used for COD measurement. Two milliliters of organic compound samples in COD cuvettes were digested in a LT 200 COD reactor. The end products were water, and carbon dioxide (Lee et al. 2011, Wu and Englehardt, 2012). As the cuvette rotated, it was measured 10 times within 5 s for an average value that eliminates any abnormal results. The reduction in COD was calculated using Eq. (4) (Wu & Englehardt 2012).

Two separated streams of distilled water and a known concentration of phenol and two parted streams of distilled water in a ratio of 2:1 were poured through the experimental rig at the start of operation without any preheating or pressurizing. The subsequent mixed solution flow was additionally diluted to a factor of 0.2. Catalyst characterization. The powdered X-ray diffraction (XRD) analysis technique was applied to determine the phase composition, crystal clear size, and structure of a solid sample. The XRD analysis was examined using a Bruker D8 Advance system (Bruker AXS, Germany) through Cu Kα radiation (λ=1.54056 A) in a 2θ range between 15° and 75°. The Scherrer method, assuming Gaussian peak broadening, was used to calculate the crystallite size of metal oxide nanoparticles (Abdpour and Santos, 2020, Al-Atta et al. 2018).

Transmission Electron Microscopy (TEM; Philips Tecnai G220) analysis was used to determine particle size and morphology based on the contrast difference of the electrons that have been transmitted through. TEM analysis was used for the characterization of metal oxide nanoparticles produced through the oxidation of phenol in supercritical water. Nanometal oxides produced in water from the oxidation experiments were allowed to settle for 24 h. A few drops of settled nanoparticles were then sampled and suspended in acetone for examination by TEM. The JEOL 2100F system (FEGTEM) was used for TEM images operating at an acceleration voltage of 100 kV (Meng et al. 2018).