DEGRADATION STUDIES ON WASTEWATER CONTAINING METHYL-DIETHANOLAMINE BY USING UV/H2O2 ADVANCED OXIDATION PROCESS
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Methyldiethanolamine (MDEA) in aqueous solutions is frequently used for scrubbing carbon dioxide (CO2) and hydrogen sulfide (H2S) from natural gas. Large quantity of MDEA disposed into the wastewater during cleaning and maintenance as well as shutdown of the absorption and desorption columns of the gas processing plant. The MDEA is not readily biodegradable and such wastewater cannot be treated using the conventional treatment facility. Advanced Oxidation Processes (AOP’s), such as oxidation by Fenton’s reagent, UV/H2O2 and UV/Ozone have been recommended as a class of techniques used for the total/partial degradation of recalcitrant organics which are not readily amenable to conventional biological oxidation. Based on the advantages of UV/H2O2 process such as no formation of sludge during the treatment, applicable for a wide range of pH, and high capability of hydroxyl radical production, the UV/H2O2 process was chosen for the treatment of effluents containing MDEA from refinery plant. For this purpose, a synthetic MDEA solution and a real effluent from gas processing unit of a refinery were used for the experiments employing UV/H2O2 advanced oxidation process. The degradation of MDEA was found to be highly dependent on the initial concentration of H2O2, the initial pH, and the reaction temperature. The important parameters that govern the MDEA degradation by UV/H2O2 process were optimized using response surface methodology (RSM). The optimum conditions for degradation process of synthetic MDEA waste were at initial pH = 9.76, ratio between contaminant to oxidant = 1000 ppm of organic carbon to 0.22 M H2O2, and temperature = 30ºC. Whilst the optimum condition of degradation process of real refinery effluent was at initial pH = 8.13, ratio between contaminant to oxidant = 1000 ppm organic carbon to 0.24 M of H2O2, and temperature = 30ºC. At optimum condition of degradation process for 3 hours irradiation using UV intensity at 12.06 mW/cm2, the maximum TOC removal achieved for synthetic and real effluent was 85.74% and 92.05%, respectively. Even though the main component of real refinery effluent was MDEA, however the other contaminats such as the presence of organic acids caused a slightly different optimum condition for degradation of refinery effluent from gas plant. During oxidation process, oxalic acid, acetic acid, formic acid, nitrite (NO2-), nitrate (NO3-), ammonium (NH4+), and carbon dioxide (CO2) were identified as the intermediates formed during degradation. Hydroxyl radical rate constants of MDEA mineralization at different temperatures by using UV/H2O2 in aqueous solution were also estimated. The rate constants of MDEA mineralization were not dependent on temperature when the temperature of reaction was less than 30ºC. Based on the estimated hydroxyl radical rate constants of MDEA mineralization at temperature 20 - 50ºC, the activation energy for mineralization of MDEA by hydroxyl radical was estimated as 10.20 kJ mol-1. The presence of bicarbonate in the solution increased the TOC removal (reached 100 % TOC removal) at an initial pH = 7. This is due to the capability of bicarbonate to act as a good buffer. At pH ≥ 7, the active site for hydroxyl radical oxidation was more provided. The biodegradability of partially degraded MDEA after UV/H2O2 was evaluated by estimation of the BOD5/COD ratio from experimental data collected, and the estimated value (BOD5/COD), proved that the partially degraded wastewater is readily biodegradable and it can safely be discharged into the environment. The energy efficiency for TOC removal of MDEA using UV/H2O2 is proved as more efficient compared to the other TOC removal technologies.