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Natural attenuation of ~(14)C-HCB and ~(14)C-DDT in Kenya paddy soil and sugarcane field soil.
Acta Pedol. Sin. 54, 108-117 (2017)
Contamination of farmland soils with persistent organic pollutants(POPs) is still a problem of environmental concerns. Take hexachlorobenzene(HCB) and 1, 1, 1-trichloro-2, 2-bis-4-chlorophenyl ethane(DDT) for example, pollution of farmland soils is generally characterized by being low in concentration and vast in area, and new pollutants flow in unceasingly from non-point sources. Farmland soils are rich in microorganisms, which potentially play a significant role in degrading such POPs. [Method] In this study, HCB and DDT were selected as representative of highly chlorinated and low chlorinated POPs, respectively, and soils were collected from a years-long paddy field and a years-long sugarcane field for use to study potentials of the soils naturally attenuating HCB and DDT. 14 C-labeled HCB and 14 C-labeled DDT were used for tracing mineralization, volatilization, metabolites, and bound residues to overcome the shortage of the traditional methods for studying the degradation of compounds. Traditional methods merely monitor the concentrations of the mother compound and/or the metabolites which may be sheltered in soil matrix via adsorption, and result in overestimating of the degradation extent. [Result] Results showed that 14 C-HCB and 14 C-DDT were spiked respectively into the paddy soil and the sugarcane field soil, and then incubated aerobically under the optimal water content (63% for paddy soil, 35% for sugarcane field soil) in an attempt to get the highest mineralization rate. Both the 14 CO 2 and the 14 C-volatiles were trapped by specific liquid in a closed system and measured with a scintillation counter. 14 CO 2 production corresponds to mineralization degree of the compounds. After 84 days of incubation, only 0.14% of 14 C-HCB and 3% of 14 C-DDT were mineralized. ASE extraction showed that penta-chlorobenzene was the only detected metabolites of HCB. DDD, DDE and DDMU were found to be the main metabolites of DDT. In the paddy soil samples, DDD was relatively higher in concentration, whereas in the sugarcane field soil samples DDE was. The extremely low mineralization extents indicate that soils are very low in potential of attenuating HCB and DDT naturally. Mere aerobic treatment is not adequate to remove HCB and DDT from soil. It is thereby inferred that it is necessary to treat the polluted soil anaerobically to remove such compounds, because in anaerobic incubation HCB would undergo reductive dechlorination which favors aerobic hydroxylation of benzene rings by dioxygenase or lignoltic enzymes, and anaerobic incubation may probably promote the growth of potential DDT degraders, like the benzene-and phenol-mineralization microorganisms that can trigger ring-cleavage reaction. Comparison between HCB and DDT in the total mineralization indicates that under aerobic conditions, high chlorinated compounds are much more persistent than the lower chlorinated compounds. HCB and DDT were quite low in volatilization, being in the range 0.1%∼0.6%, which indicates volatilization is not an important process of HCB and DDT in environment. Besides, compost from garden waste was introduced into the DDT incubation experiment to simulate effects of exogenous carbon on mineralization and volatilization of DDT. Results show that the compost increased the mineralization and the non-extractable bound residues of DDT, but reduced the volatilization of the substance; however, the effects were not statistically significant. From an engineering application view, the use of compost to enhancing DDT mineralization was not cost effective. [Conclusion]All the findings in this study may serve as reference of good reasons forunderstanding the natural attenuation of the chlorinated organic compounds in natural soils, and for remediation of soils polluted with such compounds. Since paddy soil and sugarcane field soil have their own specific dominant degradation mechanisms, it is more advisable to design case-specific strategies, anaerobic, aerobic or anaerobic-aerobic alternation, to have the pollutants degraded the most efficiently.
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Publication type Article: Journal article
Document type Scientific Article
Keywords Bound Residues ; Compost ; Mineralization ; Persistent Organic Pollutants ; Volatilization
ISSN (print) / ISBN 0564-3929
Journal Acta pedologica Sinica
Quellenangaben Volume: 54, Issue: 1, Pages: 108-117
Publisher Zhongguo Turang Xuehui zhuban
Publishing Place Bei jing
Reviewing status Peer reviewed
Institute(s) Institute of Network Biology (INET)