Substituents Transformation Mechanisms In Radicals-based Degradation Of Chlorinated Nitroaromatic Compounds

Base on the degradation mechanism of trichlorophenol by sulfate radical, chloro substituent was involved in the de novo formation of polychlorophenol through chlorine radicals addition,achieving the mutual transformation between organic chlorine and chloride ion. Similar to chloro substituent, nitro substituent, known as another common substituent, is expected to be transformed during oxidative degradation of chloronitroaromatic compounds, although its reaction mechanism remains unclear. Therefore, in the present study, Co(â…¡)/peroxymonosulfate(Co/PMS),ultraviolet/peroxymonosulfate(UV/PMS), ultraviolet/nitrite/nitrate were used as major research systems for the investigation of transformation of nitro and chloro substituents in hydroxyl/sulfate radicals(?OH/SO4?-) environment, reflecting reaction processes and degradation pathway of the chlorinated nitrocompounds in high salinity environment.The results in Co/PMS system indicated that the degradation rate of 4-chloro-2-nitrophenol(4C2NP) had a generally positive correlation with PMS initial concentration, Co2+ initial concentration, solution p H value. Additional chloride ions presented dual effects(inhibition first and then promotion) as Cl- concentration increased. However, 4C2 NP degradation was partly suppressed by nitrate ion. In Co/PMS system, dechlorination and denitration occurred along with decomposition of 4-chlorophenol/4-nitro-2-chlorophenol/4-chloro-2-nitrophenol(0.4 mmol/L4CP/4N2CP/4C2NP) according to the analysis of inorganic salt contents. The final generated Cland NO3-concentrations were 0.33 mmol/L, 0.23 mmol/L, 0.18 mmol/L and 0.21 mmol/L, 0.18mmol/L with the degradation of 4CP/4N2CP/4C2 NP, respectively. The deactivating characteristic of nitro substituent was observed in the inhibition of dechlorination and denitration processes.Additionally, the specific position of substituent in aromatic ring also played a significant role.Dechlorination, denitration denitrochlorination and re-chlorination mechanisms would be mainly involved in Co/PMS/4C2 NP system. In high salinity environment, the reaction pathways in Co/PMS/Cl- system were similar. Moreover, 2,4-dichloro-6-nitrophenol was detected by GC-MS analysis, indicating radicals and non-radicals pathways based on the chlorination of 4C2 NP.Based on ?OH/SO4?- co-presence in UV/PMS system, the results shown 4C2 NP degradation rate would be improved as a increase in illumination intensity and PMS initial concentration.4C2 NP degradation process presented slight inhibition as pH value being close to neutral, but substrate concentration mainly played an inhibitory effect. Effects of additional Cl- concentration on 4C2 NP decomposition rate kept a consistent tendency to Co/PMS system. However, inhibition effectiveness appeared a indistinctive discrepancy in low Cl- concentrations with eventual degradation rate reaching 80%. As additional NO3-concenreation increased(0~500 mmol/L),4C2 NP degradation was mainly inhibited, but the inhibition effectiveness presented an insignificant change within the tested concentration range. The degradation contribution of ?OH and SO4?-in UV/PMS system was 59.1% and 33.1%, respectively. According to analyses of kinetics and intermediates identified, the reaction mechanisms principally included denitration,dechlorination, denitrochlorination and re-chlorination processes. The previous study implied nitration of 2-nitrobenzene in ?OH-based system, but re-nitration process involved in the mutual transformation between nitrate ion and nitro substituent was inhibited due to the key nitrating agent NO2?being oxidized by PMS(E0(HSO5-/HSO4-)=1.85 V vs NHE).Based on photochemistry nitrite and nitrate, the results indicated that 4C2 NP degradation rate was accelerated under strongly acidic condition and the highest decomposition rate was achieved in UV/NO2-/NO3-system compared to other comparison systems. Meanwhile, 4C2 NP degradation rate was higher than 2NP and 4N2 CP in UV/NO2-/NO3-system under the same pH condition.According to degraded intermediates identified and kinetics analysis, chloro substituent had an adverse effect on benzene activation in contrary to nitro substituent. Nitration, chlorination,oxidization and re-chlorination were involved in 4CP degradation process, which was similar to the specific reaction mechanisms of 4C2NP/4N2CP/2NP in UV/NO2-/NO3-system.