Chapter 4 Results and discussion
4.3 Determination of carbofuran degradation pathway
The intermediate compounds formed in the Fenton degradation of carbofuran in the aqueous solution after 20 min were analyzed by GC-MS. Four products were identified by the molecular ion and mass fragment ions, and were compared with NIST library data. Table 4-5 summarizes the molecular weights of these products.
From mass spectrum, the product retention time of 12.88 min was found to be carbofuran, the parent compound. By interpreting the mass spectrum, the product with a retention time of 7.44 min was identified as 2,2-dimethyl- 2,3-dihydro-benzofuran-7-ol, which is the product due to the cleavage of the carbamates group from carbofuran. This product was also detected in the hydrolysis (Wei et al., 2001), photolysis (Bachman and Patterson, 1999), TiO2 photocatalysis (Kuo and Lin, 2000) and AFT (Wang et al., 2003) of carbofuran. The product with a retention time of 13.80 min was identified as 7-hydroxy-2,2-dimethyl-benzofuran-3-one. This was formed by further oxidizing 2,2-dimethyl-2,3-dihydro-benzofuran-7-ol on the furan ring. Wang et al. (2003) also reported these products as intermediates of carbofuran under AFT. The product retention time of 3.6 min was attributed to the 1,4-Benzenedicarboxaldehyde anion, which was formed by opening the furan ring and demethylation from 7-hydroxy-2,2-dimethyl-benzofuran-3-one. Further degradation products formed if any are not detected in the present study, which can be attributed to their low concentration, low extraction efficiency and/or limited sensitivity of the GC-MS. The intermediate products of carbofuran identified in the present study are listed in Table 4-5 and the possible carbofuran degradation pathway is proposed in Figure 4-17.
58
Table 4-5 Intermediate products of carbofuran identified by GC-MS
Product name
Molecular
Weight Formula
Retention time (min)
1,4-benzenedicarboxaldehyde anion 134 C8H6O2 3.60
7-benzofuranol, 2,3-dihydro-2,2-dimethyl 164 C10H12O3 7.44
Carbofuran 221 C12H15NO3 12.88
7-hydroxy-2,2-dimethyl-benzofuran-3-one 178 C10H10O3 13.80
59 O
O C NH
•OH
CH3
CH3
CH3
O OH
•OH CH3
CH3
O OH
•OH CH3
CH3
O
O OH
(m/Z = 221)
(m/Z = 164)
(m/Z = 178)
(m/Z = 134)
1,4-benzenedicarboxaldehyde anion 7-benzofuranol, 2,3-dihydro-2,2-dimethyl
Carbofuran
7-benzofuranol, 2,3-dihydro-2,2-dimethyl
Figure 4-17 Carbofuran degradation pathway in the Fenton process
60
Chapter 5
Summary and conclusions
Based on the extensive experimental investigations, the following conclusions can be drawn.
1. Oxidation with Fenton’s reagent has proven to be an efficient and fast process for degrading carbofuran in contaminated water. Significant percentages of carbofuran removal were achieved in just 20 min of reaction time.
2. Carbofuran degradation was strongly affected by the initial concentrations of H2O2 and Fe2+. The central composite design and response surface methodology indicate that the optimal Fe2+ and H2O2 concentrations are 9.5 and 217 mg/L, respectively for an initial carbofuran concentration of 50 mg/L at pH 3. Similarly, the optimal Fe2+ and H2O2 concentrations are 7.4 and 143 mg/L, respectively for an initial carbofuran concentration of 10 mg/L at pH 3.
3. The initial concentration of carbofuran was an important parameter affecting the efficiency of pesticide removal by the Fenton process. Increase in initial carbofuran concentration decreased the overall carbofuran removal percentage.
Complete degradation of 10 mg/L of carbofuran was achieved after 20 min of reaction under the optimum conditions of Fenton process.
4. Carbofuran was degraded into 7-benzofuranol, 2,3-dihydro-2,2-dimethyl, 7-hydroxy-2,2-dimethyl-benzofuran-3-one and 1,4-benzenedicarboxaldehyde anion. The other degradation intermediates of carbofuran formed in the system if any are not identified. Hence, the further study on the identification of complete carbofuran mineralization pathway may be an interesting future research goal.
61
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