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Transformation of Organic Pollutants in Enzyme Catalyzed Oxidative Processes

Author: XiaQing
Tutor: LuZuoHe
School: Nanjing Agricultural College
Course: Environmental Science
Keywords: Humification Catalyzed oxidative reactions Chloroanilines Estrogens Horseradish peroxidase (HRP) Laccase
CLC: X703
Type: Master's thesis
Year: 2013
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Abstract


Enzyme catalyzed oxidative coupling reactions constitute an important class of reactions involved in natural humification processes. The reactions also affect the transformation, mobility, and bioavailability of certain organic pollutants, leading to their coupling with soil organic matter (SOM) to form non-extractable bound residue. Knowledge regarding the underlying reaction mechanisms and the stability of coupling products is not only of relevant in assessing the ecological hazards of the pollutants but also lays a foundation for developing a pollution control technology involving enzyme catalyzed humication reactions.In this study, we systematically investigated that reaction mechanisms, kinetics, and products of chloroaniline in horseradish peroxidase (HRP) catalyzed systems in the presence of model humic constituents. It was demonstrated that among the variety of model humic constituents, only syringic acid and catechol significantly facilitated the removal of pollutants. However,2-chloroaniline coupling with the2humic constituents was via slightly different pathways.1,2-neucleophilic addition and1,4-neucleophilic addition are involved with syringic acid and catechol, respectively. The latter showed empirical2nd order kinetics but no clear kinetic trend appeared in the former pathway. The transformation of2-chloroaniline was solely contributed by the first reaction step between catechol and HRP in1,4-nucleophilic addition pathway. The1,2-nucleophilic addition between2-chloroaniline and the quinonic intermediate was more rate-limiting than the preceding oxidation step. In addition, the cross-coupling products formed via2different pathways showed different stability.1,4-addition pathway seemed to be more permanent than1,2-addition pathway. Short-term stability experiments demonstrated that cross-coupling products with syringic acid which could be easily released in acidic conditions. However, cross-coupling products with catechol were relatively stable at the same condition.We also explored the application of enzyme catalyzed process in pollution control. Steroid estrogens were selected as the target of treatment. The removal of5estrogens (bisphenol A, estradiol, ethinylestradiol, estrone, octylphenol) in laccase and HRP catalyzed oxidative processes was studied with emphasis on the effects of pH, enzyme activity and NOM on their removal. In addition, EE2was selected as a typical estrogen to investigate the reaction kinetics and mechanisms in laccase catalyzed oxidative processes. In laccase catalyzed system, the results indicated that laccase was capable of removing estrogens efficiently. The optimal pH was between4and6. NOM significantly inhibited the removal of estrogens in the initial stage of the reaction. However, the adverse effect of NOM was not obvious after24h treatment. The removal of EE2was second-order in kinetics with first-order to both the concentrations of the enzyme and contaminant. MS analysis demonstrated that EE2dimer formed through radical coupling mechanism is the main reaction product. In HRP catalyzed system, the results indicated that HRP was also capable of removing estrogens efficiently. The influence of pH on the performance of the HRP was not significant at the selected reaction conditions. Similar removal rates were achieved at pH4～8. Almost complete reduction could be achieved at low HRP dosage. The presence of NOM showed little adverse effect on the removal of the contaminants. Compare with the two results, although the influences of pH and NOM on HRP activity was smaller than that of laccase, the stability of laccase was higher than that of peroxidase.

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CLC: > Environmental science, safety science > Processing and comprehensive utilization of waste > General issues > Wastewater treatment and utilization
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