Degradation Intermediates Identification and Development of

95

4.2 Degradation Intermediates Identification and Development of

Degradation Mechanism Degradation of aqueous MDEA solution using UVH 2 O 2 has been studied. MDEA C 5 H 13 O 2 N is a compound containing the elements namely C, H, O, and N and the structure is shown in Figure 2.1. For the identification of degradation intermediates, experiments were conducted using the following conditions: [MDEA] = 2000 ppm = 1000 ppm TOC, [H 2 O 2 ] = 0.22 M, initial pH = 9.76, temperature = 30 ⁰C, UV intensity = 12.06 mWcm 2 , and reaction time = 3 hours. The end products after the complete degradation of MDEA containing these elements, using hydroxyl radical HO • is expected to be CO 2 , H 2 O, NH 3 NH 4 + , NO 2 - , NO 3 - , and N 2 . Based on the analysis of the degradation intermediates obtained during the degradation of MDEA, using UVH 2 O 2 process, the identified organic by-products are: oxalic acid C 2 H 2 O 4 , acetic acid CH 3 COOH, formic acid HCOOH, and the inorganic by-product are: nitrate NO 3 - , nitrite NO 2 - , ammonium NH 4 + , and carbon dioxide CO 2 . Figure 4.19 shows the profile of inorganic by-products during UVH 2 O 2 process. Figure 4. 19 Inorganic by-product profile during the UVH 2 O 2 process. 96 It can be seen that the concentration of ammonium and nitrite increased until 90 minutes and then decreased with reaction time. Oxidation of both ammonium and nitrite by hydroxyl radical resulted in the production of nitrate. The concentration of nitrate increased along with the decrease in ammonium NH 4 + and nitrite NO 2 - concentration. Similar by-products such as ammonium and nitrate were also identified and reported by Klare et al. [89] and Low et al. [87]. Klare et al. identified NH 3 NH 4 + , nitrate NO 3 - , and nitrite NO 2 - as intermediate products in the partially degraded secondary amine. Low et al. [87] also found ammonium NH 4 + and nitrate NO 3 - during the degradation of organic compound containing nitrogen atom. Both researchers used TiO 2 mediated photo-catalytic technique for degradation. Many other researchers who identified the inorganic by-products containing nitrogen atom during their study on AOP for the degradation of organic compound containing nitrogen atom are listed in Table 2.10. Identification of organic by-products during the degradation of MDEA using UVH 2 O 2 was conducted using high performance liquid chromatography HPLC. The chromatogram obtained using YMC-Pack PolymerC18 column is shown in Figure 4.20. After 30 minutes of radiation, one of the intermediate products was identified as oxalic acid, which appeared at 3.9 minutes of retention time. Other by- products could not be identified using this column and hence Transgenomic column was used, which show the presence of two more organic acids. Those two by-products were identified as acetic acid RT = 8.1 min and formic acid RT = 8.9 min, respectively Figure 4.21. Organic by-products profile during UVH 2 O 2 process is presented in Figure 4.22. As reported by many researchers [60, 89, 143], degradation of organic compound containing amino group by using hydroxyl radical is highly dependent on pH. At low pH, free electron pair of nitrogen atom was protonated, and resulting in the deactivation of α – CH bond [33, 89]. Hence, the hydroxyl radical can only attack the further location of CH bond after α – CH bond. Meanwhile, at any pH ≥ 7, free electron pair of nitrogen atom was un-protonated. In this condition, hydroxyl radical can attack the free electron pair of nitrogen atom and the α – CH bond [144 – 145]. However, steric effect of components attached to the nitrogen atom was also 97 identified. The steric effect drives the ability of hydroxyl radical to attack the electron pair of nitrogen atom [89]. Figure 4. 20 Chromatogram of partially degraded MDEA after UVH 2 O 2 process using YMC-PolymerC18 column. 98 Figure 4. 21 Chromatogram of partially degraded MDEA after UVH 2 O 2 using Transgenomic column. 99 Figure 4. 22 Organic by-products profile during the UVH 2 O 2 process. Based on the observation of inorganic Figure 4.19 and organic by-products Figure 4.22 profiles, initially formic acid was formed as a by-product. The proposed mechanism of MDEA mineralization by hydroxyl radical can be explained as follows. Initially hydroxyl radical was generated by H 2 O 2 photolysis. MDEA consist of two groups of ethanol and one methyl. During the mineralization of MDEA by hydroxyl radical, initially hydroxyl radical attacked α – CH bond of methyl group by taking one H to form water H 2 O [31] and a radical compound, through hydrogen abstraction Figure 4.23, scheme 1. Further, the hydroxyl radical attacked the organic compound radical electrophilic addition [32] and then cleaved the C-N bonds. During the next step, formic acid was produced. Acetic and oxalic acids appeared later. The formation of these two organic acids occurred by cleavage of C-N bonds of the ethanol groups attached to the nitrogen atom of MDEA [89]. Further oxidation of by-products by hydroxyl radical Figure 4.24, schemes 2, 3, 4, 5, and 6 would result in the formation of few end products such as ammonium NH 4 + , nitrate NO 3 - , nitrite NO 2 - , and carbon dioxide CO 2 . Figure 4.23 and 4.24 show the proposed mineralization mechanism of MDEA by hydroxyl radical. 100 Figure 4. 23 The initial stages of MDEA mineralization by hydroxyl radical. 101 Figure 4. 24 Reaction pathways of intermediate degradation by hydroxyl radical. 102

4.3 Kinetics of MDEA Mineralization Process