Photochemical oxidation of methyldiethanolamine (MDEA) in aqueous solution by UV/K2S2O8 process
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Methyldiethanolamine (MDEA) as an organic material is a hazardous contaminant in the aquatic environment because of its adverse effects on aquatic life, environment, and humans. In this study, a batch reactor of ultraviolet (UV) light and peroxydisulfate was performed to investigate the degradation of MDEA in aqueous media. The effect of different experimental parameters such as UV irradiation, peroxydisulfate concentration, MDEA concentration, temperature, and solution pH on removal of MDEA was evaluated precisely. No significant degradation was observed with a separate UV light. Adding peroxydisulfate to the solution increased the removal performance more than 75 %.
KeywordsPeroxydisulfate MDEA Advanced oxidation processes Wastewater treatment
Raw natural gas includes some acidic gases such as H2S and CO2. These acidic gases are very corrosive and toxic to the environment, and therefore required to be removed. Different alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA) and diisopropanolamine (DIPA) are used for the removal of acidic gases in the sweetening gas units . In addition N-methyldiethanolamine (MDEA) as metal-MDEA complexes have as property significant ultraviolet (UV) absorption. New photosensitive precursors was prepared as thin films by N-methyldiethanolamine complex [2, 3]. Usually during cleaning, protecting and scheduled control of absorption and desorption column, high concentration of alkanolamine is generated into the wastewater . Nevertheless, due to its toxicity the conventional biological treatment cannot be used for this wastewater .
During recent two decades, advanced oxidation processes (AOPs) have been considered as popular techniques to treat the high concentration of organic contaminant in the wastewater . AOPs are of the most alternative techniques for destruction of many other organic matters in wastewater and effluents. These processes generally involve UV/H2O2, UV/O3, UV/S2O8 2− or UV/Fenton’s reagent for degradation of contaminants [7, 8, 9].
A large number of experimental works have been performed on the application of AOPs to treat wastewater. The Fenton’ reagent in the AOPs was used to degrade MEA , DEA , N,N-diethyl-p-phenylenediamine  and DIPA . Also, the use of UV/H2O2 in the AOPs for degradation of MEA and MDEA [4, 14] and ozonation for degradation of DEA  have been studied.
Because of its high reactivity of UV/S2O8 2− process, high solubility, relatively low cost of peroxydisulfate and benign end products, recently the application of UV/S2O8 2− in wastewater treatment was investigated in numerous studies . Peroxydisulfate (S2O8 2−) is a strong oxidant (E 0 = 2.05 V) which has been used widely in the petroleum industry for the treatment of hydraulic fluids or as a reaction initiator .
As can be seen in the above reactions, the oxidation process is begun by production of the sulfate and hydroxyl radicals (Eqs. 1 and 2). These radicals are powerful oxidizing agents which may attack the organic matters (R) in the contaminated water. It causes, ultimately, complete decomposition of toxic and bioresistant compounds to harmless species (like CO2, H2O, etc.). Sulfate ion will be generated as the end product, which is practically inert and is not considered to be a pollutant. It is worth to mention that the United States Environmental Protection Agency (USEPA) has listed SO4 2− under the secondary drinking water standards. A maximum concentration of sulfate ion is 250 mg l−1 (1.43 mM), based on sanitary reasons such as taste and odor [16, 21].
The peroxydisulfate is normally available as a salt associated with ammonium, sodium, or potassium. The comparative performance of K2S2O8 (KPS) and (NH4)2S2O8 (APS) as an oxidant under the irradiation of UV light for removal of butylated hydroxyanisole , the dye Reactive Yellow 84  and tylosin  was investigated. The results have indicated that KPS provides a more rapid photooxidative removal than APS at neutral pH. The difference in the removal efficiency is apparently due to the presence of the ammonium ion. The aqueous ammonium can undergo photooxidation leading to nitrate and/or nitrite by the available oxidants in the solution, such as, S2O8 2−, and its related intermediates H2O2 or O2 [19, 20]. Furthermore, the reaction of NH4+/NH3 with UV/S2O8 2− process is proved to be able to convert it to nitrate under the 254 nm photolysis , thereby making the ammonium as a competitor of the organic pollutants. In view of this, and the general unsuitability of adding ammonia to waters, APS is not recommended to be used in the UV/peroxydisulfate oxidation process. Therefore, the UV/KPS combination was chosen for further investigation throughout this study.
It has been proven that UV/S2O8 2− and UV/H2O2 (the most common process) have similar reaction rate constants . Moreover, peroxydisulfate advantages UV/H2O2 and other similar approaches by the following reasons: (1) Peroxydisulfate ions seem to be more useful when the process is not well controlled, for example when overdosing occurs because of the potential quenching effect of using H2O2 . (2) Peroxydisulfate would be more applicable for industrial uses in comparison to liquid oxidants such as H2O2, because it is a solid oxidant. (3) Peroxydisulfate salts are much cheaper than other oxidants like hydrogen peroxide and ozone [26, 27, 28].
In this study, using UV/S2O8 2− process the destruction of MDEA as an amine pollutant from contaminated water was investigated. Moreover, effect of different experimental parameters such as UV irradiation, peroxydisulfate concentration, MDEA concentration, Temperature, and pH was evaluated.
Potassium peroxydisulfate (K2S2O8), sulfuric acid and sodium hydroxide were of laboratory reagent grade (Merck Co., Germany) and used without further purification. The synthetic wastewater for which treatment process was performed contains methyldiethanolamine (MDEA).
Methyldiethanolamine (MDEA) degradation experiments were conducted in a photoreactor. For UV/peroxydisulfate process, irradiation was carried out with a 125 and 250 W (UV-C) mercury lamp (Philips, the Netherland), which was put above a batch photoreactor. The distance between the solution and UV source was adjusted according to the experimental conditions. The volume of sample was 500 ml, and total time of experiment was 60 min. The reactor had a water-flow jacket for regulating the temperature by means of an external circulating flow of a thermostat. Since the photocatalysis is sustained by a ready supply of dissolved oxygen, air was supplied to the reaction system at a constant flow rate using a micro-air compressor.
Results and discussion
Effect of peroxydisulfate and UV irradiation on degradation of MDEA
Both SO 4 ∘− and OH∘ are possibly cause of the degradation of organic contaminants. Meanwhile either radical may predominate over the other depending on pH conditions, and react with organic compounds commonly by three mechanisms: hydrogen abstraction, hydrogen addition, and electron transfer.
Sulfate radicals show a higher standard reduction potential than hydroxyl radicals at neutral pH, and both radicals show similar reduction potentials under acidic conditions . In general, SO 4 ∘− is more likely to participate in electron transfer reactions, whereas OH∘ is more likely to participate in hydrogen abstraction or addition reactions .
Effect of initial MDEA concentration
Effect of initial peroxydisulfate concentration
Investigations were made using varying the concentration of S2O8 2− from 5 to 25 mM at fixed initial MDEA concentration of 500 ppm, pH = 8.5, irradiation of 250 W and temperature of 30 °C. Studies have revealed that increase in amount of S2O8 2− from 5 to 25 mM would enhance degradation of the MDEA . These observations can be explained by the fact that the increase in concentration of peroxydisulfate results in higher generation of hydroxyl and sulfate radicals and improves the photooxidative degradation of the MDEA consequently. It is likely because of excessive generation of hydroxyl radicals (Eqs. 1 and 6) that would be recombined to less reactive form of H2O2 (Eq. 10), which is a known quencher of OH∘ radical (Eq. 11). Therefore, the destruction of MDEA was slightly slowed down at higher S2O8 2− dosages. However, such a recombination effect of the radical was likely not very effective due to the low steady-state concentrations of the radicals; higher decay rates of MDEA at higher S2O8 2− dosages are still expected [20, 34]. For example, Lin et al. investigated the degradation of ciprofloxacin by UV/peroxydisulfate process. They described the removal efficiency increased with an increase for S2O8 2− . They described the degradation efficiency of ciprofloxacin increased with time, reaching 95 % after 30 min.
The first-order reaction kinetics was used to study the degradation kinetics of MDEA by UV/S2O8 2− process. The individual expression was presented as below:
Rate constants of UV/peroxydisulfate oxidation of MDEA at 30 °C
[S2O8 2−]0 (mM)
k 1 (min−1)
Effect of UV irradiation
Effect of the initial pH
Effect of the temperature
Temperature helps the degradation reaction to compete more effectively according to the Arrhenius equation; however, at the same time, it reduces the oxygen solubility in water which is not desirable .
Almost no MDEA removal was achieved using UV irradiation alone and using peroxydisulfate alone removal percentage was obtained about 7 %. Finally, more than 75 % of MDEA concentration removed using UV irradiation and peroxydisulfate simultaneously.
Increase in MDEA concentration would decrease the degradation. For example, the degradation of 50 % was observed in 500 ppm, while only degradation of 19 % was obtained in 1500 ppm concentration of MDEA.
In terms of the changed of peroxydisulfate concentration, i.e., 0 to 25 mM, the COD removals increased.
The COD removal increased as temperature and UV irradiation intensity increased.
The solution pH had a major influence on MDEA degradation in the UV/K2S2O8 process. The range of optimum solution pH was 6–7; however, all levels of pH demonstrate satisfactory removal but these ranges can be better for industrial conditions.
Increase in the temperature would increase the degradation according to the Arrhenius equation. Increasing UV irradiation intensity increases removal by improving production of sulfate and hydroxyl radicals.
The optimum operating conditions, which showed that the initial MDEA concentration of 500 ppm, UV irradiation of 250 W, initial peroxydisulfate concentration of 25 mM, and a temperature of 30 °C were the best conditions. Under the optimized conditions, the maximum degradation of MDEA was 75 %.
The authors thank the department of chemical engineering in Razi university of Kermanshah, Iran, for financial and other supports.
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