Treatment of Ethanolamine Wastewater using Granular Activated Carbon as Particle Electrodes

Abstract

Ethanolamine (monoethanolamine, ETA, C2H7NO) is an organic chemical compound containing amino and alcohol functional groups. According to MSDS descriptions, ethanolamine is harmful by inhalation and in contact with skin. It causes eyes burns, skin burns and dermatitis. The combustion of ethanolamine produces harmful and irritant gas which becomes the cause of air pollution. Ethanolamine increases COD and T-N when released into water. Ethanolamine is commonly used to remove sour gases (e.g., H2S and CO2) from natural gas, a process known as natural gas sweetening (Ndegwa et al., 2004). Ethanolamine can absorb CO2 from combustion gases so that it has gained attention for the abatement of greenhouse gases. Ethanolamine is also widely used for alkalinization of water in steam cycles of nuclear power plants with pressurized water reactor (PWR). The steam cycles of nuclear power plants use an ion-exchange resin column to capture ethanolamine and it is released into the wastewater through a resin regeneration process (Kim et al., 2010). The disposal of the ethanolamine contained wastewater is a problem because it could increase COD and T-N concentrations. Thus, treatment of ethanolamine contained wastewater is required. Ethanolamine contained wastewater has been treated by ion-exchange resin, biodegradation, and oxidation processes. Chi and Rochelle (2002) have been studied oxidation of monoethanolamine using Fe3+ as catalysist. However, these methods require large amounts of chemicals, high cost and degradation reaction proceeds longer time. Therefore, an alternative technology process is required. Three-dimensional electrodes reactor (TDE) consists of a number of particles which are packed between two feeder electrodes. TDE has higher specific surface area than two-dimensional electrodes reactor. Because many small particles placed into the TDE system form charged microelectrodes which are called particle electrodes under the influence of electric field. Thus, the use of TDE has been emphasized as it can yield higher treatment efficiency. GAC is used in many electrochemical applications, because GAC has high electrical conductivity and capacitance. It was predicted that TDE can degrade COD and T-N by both electrochemical oxidation and electro adsorption. In this research, using the GAC as particle electrodes was expected to keep high COD and T-N reducing efficiency. The objective of this research is to investigate the applicability of GAC as particle electrode for treat ethanolamine wastewater in TDE system. To achieve this goal, first, ethanolamine adsorbability of GAC was examined. Adsorption capacity of GAC was found to be 0.7 mg COD/g GAC and 3.4 mg T-N/g GAC, the ethanolamine adsorption of GAC could not proceed. It may be due to the ionic repulsion between the negatively charged activated carbon and ethanolamine solution. Therefore, in this study, GAC as particle electrode in the TDE was examined. Experiments were conducted to determine various parameters affecting the performance of the treatment of GAC-TDE, such as packing ratio, applied cell voltage, location of feeder electrode and initial pH of ethanolamine solution. First, effect of the packing ratio (1:1, 1:2, 1:4, particle electrodes : insulators, V:V) was examined. In order to compare the applicability of particle electrodes, ZVI used by Jeong et al. (2012) was examined. The maximum of COD and T-N reducing ability was found at the packing ratio of 1:1 GAC-TDE, the concentrations of COD and T-N were decreased to 4 mg/L (C0 = 838 mg/L) and 2.16 mg/L (C0 = 144 mg/L). At the ratio of 1:2, GAC-TDE could reduce COD concentration from 838 mg/L to 3 mg/L and ZVI-TDE could reduce to 784 mg/L. 1:1 GAC-TDE was decreased to 4 mg/L and 1:4 GAC-TDE was decreased to 49 mg/L, respectively. COD reducing ability of GAC-TDE was very higher than ZVI-TDE. A high reduction will be attributed to that the GAC as particle electrode can reduce COD and T-N by both electrochemical oxidation and electro sorption (Wang, L. et al., 2007). Maximum current efficiency was 15.77% of the ratio of 1:4 GAC-TDE; 7.67% of 1:2 GAC-TDE; 6.16% of 1:1 GAC-TDE and 5.86% of ZVI-TDE. GAC-TDE has a higher COD and T-N reducing efficiency than the conventional three-dimensional electrode reactor (e.g., ZVI-TDE) and GAC adsorption bed. Second, it was expected that the GAC can electro adsorb of ethanolamine while the electrochemical reaction in TDE. Water desorption experiment was conducted using GAC-TDE at the packing ratio 1:1. 250~350 mg/L of COD and 0.4~7.6 mg/L of ammonia were detected at the start. It is indirectly ascertained that COD and T-N electro adsorption onto GAC. Third, experiments of applied cell voltages (50, 100, 300 and 600 V) to TDE system were performed. Experiments were carried out at packing ratio 1:2 GAC-TDE. It has been known that electrochemical degradation of organic pollutant was increased at the higher cell voltage, because high cell voltage could accelerate electrochemical reaction (Xiong et al., 2003). However, results of present study indicate that COD reducing efficiency in the 300 V was higher than 600 V. When a cell voltage of 300 V was applied, concentration of COD and T-N were decreased to 3 mg/L (C0 = 838 mg/L) and 4.29 mg/L (C0 = 159 mg/L), respectively. When a high cell voltage was applied, current was increased with treatment time due to the increase of side reaction, such as evolution of oxygen and hydrogen and generation of heat. Therefore, it is necessary to limit the current of operation to gain a high current efficiency. Fourth, impact of the location of feeder electrodes when ethanolamine treated with GAC-TDE was examined. A-TDE (anode bottom and cathode top) and C-TDE (cathode bottom and anode top) were examined at the packing ratio 1:2 GAC-TDE, 300 V. The COD concentration was decreased from 838 mg/L to 3 mg/L at A-TDE, but at the C-TDE, COD was decreased to 171 mg/L. Before the experiments, it was expected that reduction of COD and T-N is not affected location of feeder electrode, because GAC particle behaves as an anode on one side and a cathode on the other side in TDE system. However, results show a difference due to the influences of pH of ethanolamine solution. Lastly, results of these experiments was indicated that pH of ethanolamine solution affects the ethanolamine reducing efficiency in GAC-TDE. Therefore, impact of the initial pH of ethanolamine solution (pH 3, 7 and 10) was examined at the packing ratio 1:2 GAC-TDE, 300 V. The maximum of COD and T-N reducing ability was found at initial pH 10, the concentration of COD and T-N were decreased to 1 mg/L (C0=838 mg/L) and 1.22 mg/L (C0=154 mg/L), respectively. When the initial pH is high, COD and T-N reducing efficiency was increased due to the hydrogen evolution and effect of chemical affinity. This result is consistent with the previous experiments. This research was evaluated the effectiveness of GAC as particle electrode. Using the GAC as particle electrode will provide a better ETA reduction with reducing electrical potential dissipation of an electrochemical process. The use of GAC may be beneficial as it can lead to higher ethanolamine reducing, it is potential to application for treatment of ethanolamine contained wastewater.