Supercritical water oxidation of wastewater from LCD manufacturing process: kinetic and formation of chromium oxide nanoparticles

Supercritical water oxidation (SCWO) has been known as one remarkably effective process for the treatment and disposal wastewater. In this work, SCWO of the wastewater from a liquid crystal display (LCD) manufacturing plant was carried out in an isothermal, isobaric tubular flow reactor with H2O2 as an oxidant. All experiments were performed at pressures from 25 to 29 MPa and at temperatures from 396 to 615 °C. At the reactor entrance, the initial chemical oxygen demand (COD) concentrations of wastewater were between 126 and 19,428 mg/l; the oxidant concentrations were between 4.66 × 10−3 and 1.34 × 10−1 mol/l. During the SCWO process, in situ formation of chromium oxide nanoparticles (α-HCrO2 and Cr2O3) was found by decomposition of ammonium dichromate, which is contained in wastewater. Based on scanning electron microscope and electrophoretic light scattering analyses for particle size, non-faceted crystalline chromium particles are obtained in the range 200-400 nm. The COD conversion rate was increasing with higher wastewater and oxidant feed concentration. An assumed first-order global power-law rate expression was determined with activation energy of 21.71 ± 1.67 kJ/mol and a pre-exponential factor of 46.84 ± 1.52 s−1 to a 95% confidence level. By taking into account the dependence of the reaction rate on wastewater and oxidant concentration, a global power-law rate expression was regressed from the complete set of data. The resulting activation energy were 47.79 (±1.52) kJ/mol; the pre-exponential factor was 2.78 (±0.71) × l02 l1.075 mmol−0.075 s−1; and the reaction orders for wastewater (based on COD) and oxidant were 1.01 ± 0.01 and 0.065 ± 0.01, respectively.