肯尼亚水稻土和甘蔗地土壤中14C-六氯苯和14C-滴滴涕的自然消解

Natural Attenuation of ^(14)C-HCB and ^(14)C-DDT in Kenya Paddy Soil and Sugarcane Field Soil

氯代持久性有机污染物的农田土壤污染呈现污染浓度低、面积大、新源污染不断输入的特点。农田土壤本身微生物种类丰富,对氯代有机污染物具有较大的降解潜力和未知性。本试验以典型高氯代和低氯代持久性有机污染物——六氯苯(HCB)和滴滴涕(DDT)为研究对象,结合^(14)C同位素示踪技术,研究HCB和DDT在热带水稻土和甘蔗地土壤的矿化现象,同时监测HCB和DDT在两种土壤中的挥发、降解产物以及结合残留。结果表明,经84 d好氧培养,HCB和DDT在两种土壤中的矿化量分别仅为0.14%和3%,低氯代有机污染物DDT的矿化速率显著高于高氯代有机污染物HCB。然而,两种土壤对HCB或DDT的矿化没有显著性差异。HCB或DDT在水稻土中的挥发量略微高于甘蔗地土壤,两种土壤中HCB和DDT的挥发量在0.1%~0.6%之间,表明挥发不是其主要的环境过程。在DDT污染水稻土和甘蔗地土壤中添加1.25%的堆肥增加了DDT在土壤中的矿化与结合残留,减少了DDT的挥发。本研究结果表明土壤在好氧条件下对氯代持久性有机污染物的自然消解能力非常弱,而有机肥的使用有助于土壤中持久性氯代有机污染物的矿化消除。

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. 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. 14C-labeled HCB and 14C-labeled DDT were used for tracing mineralization, volatilization, metabolites, and bound residues to overcome the shortage of the traditional methodsfor 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. 14C-HCB and 14C-DDT were spiked respectively into the paddy soil and the sugarcane field soil, 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 14CO2 and the 14C-volatiles were trapped by specific liquid in a closed systemand measured with a scintillation counter. 14CO2 production corresponds to mineralization degree of the compounds. After 84 days of incubation, only 0.14% of 14C-HCB and 3% of 14C-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. All the findings in this study may serve as reference of good reasons for understanding 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.