Microbial And Photocatalytical Degradation Of Endosulfan In Soil With The Solubilization Of Surfactants

Endosulfan is a widely used organochlorine pesticide (OCP) to control many insects and mites for field crops around the world. Owing to its highly toxic and bioaccumulation to most living organisms, and long-distance transport from its original source in the environment, endosulfan was listed in persistent organic pollutants (POPs) by Stockholm Convention in2011. With the long-term use of endosulfan. more and more endosulfan and its highly toxic metabolite (endosulfan sulfate) have been accumulated in agricultural soil in China, which poses a serious threat to agricultural ecological environment and human health. With being banned to use endosulfan gradually, people begin to concern on soil pollution problem in discarded manufacturing enterprise and storage site. China is a party to the Stockholm Convention on POPs control, and has responsibility and obligation to control the use of endosulfan. to prevent environmental pollution and develop the remedation technology for endosulfan-contaminated soil.At present, microbial degradation is widely used to remove endosulfan in soil. However, endosulfan is a hydrophobic organochlorine pesticide, and can be solidly adsorbed by soil colloids, which leads to its low bioavailabilitv. and then affact the degradation efficiency of microorganisms. As a toxic metabolite of microbial degradation, endosulfan sulfate can be accumulated in soil for a long time, which may limit the degradation efficiency of microorganisms and practical application of this method. So these studies are still restricted to the laboratory. How to increase the solubility and bioavailability in soil and simultaneously remove endosulfan sulfate becomes an important topic for the development of remediation technology for endosulfan-contaminated soil and improvement the remediation efficiency. On the other hand, endosulfan in soil is difficult to degrade directly by sunlight, and few reports on the photocatalytic degradation of endosulfan in soil can be found. However, it was reported that endosulfan could be degraded thoroughly in aqueous phase by photocatalyst. This research inspires a new idea to degrade endosulfan in the contaminated soil, which is worthy to study using photocatalytic degradation.In this thesis, we tried to improve the solubility and bioavailability of endosulfan in soil by screening suitable surfactants and their combination. An endosulfan degradading bacterium was isolated from the contaminated aged soil, and was employed to degrade endosulfan in soil with the elution of surfactants. Meanwhile, fluorine and nitrogen-doped TiO2which may effectively catalyze endosulfan and its metabolites to degrade were synthesized by sol-gel method, and were used to degrade endosulfan in soil with the elution of surfactants. With the solubilization of surfactants, we tried to integrate microbial degradation and photocatalytic degradation techniques, and hoped to obtain the combination degradation method for endosulfan in the contaminated soil, and to provide a scientific basis for developing the theory and technology to remedy the contaminated soil. The main results are summarized as follows.1. Surfactants and their combination which could significantly improve the solubility of endosulfan and elute efficiently the contaminated aged soil were obtained. It could be found that the solubilities of a-endosulfan in Tween80, Triton X-100and SDS (sodium dodecyl sulfate) with the concentration of200～600mg·L’1were9.28～25.70,3.95～8.40and0.88～1.83mg·L-1, respectively, which were negatively correlated with critical micelle concentrations (CMCs) of these surfactants, while the solubilities of β-endosulfan in them showed the similar trend as that of a-endosulfan. When the mass ratio of anionic and nonionic surfactants was not less than4:1, Tween80/SDS and Triton X-100/SDS could synergistically increase the solubility of endosulfan, and the solubilization capacity of the former was stronger than the latter. The solubilization capacities of the surfactants followed a decreasing order of Tween80, Tween80/SDS, Triton X-100, Triton X-100/SDS, and SDS, and those could further increase with the similar trend in the presence of1000mg·L-1Na2SiO3.The elution percents of α-,β-endosulfan followed a decreasing order of Tween80/SDS, Tween80and Triton X-100in the absence of Na2Si03, while that of Triton X-100/SDS at low concentration (100～500mg·L-1) and high concentration (800～1000mg·L-1) were lower and higher than those by the corresponding concentrations of Triton X-100, respectively. In general, the elution capability of Triton X-100did not obviously improve with the addition of SDS. The elution percents of enduslfan follwed a decreasing order of Tween80/SDS, Tween80, Triton X-100/SDS, and Triton X-100in the presence of Na2SiO3, and those of a-endosulfan were as1.17～2.73,1.87～4.02,1.85～6.56and1.87～2.85times as that in the absence of Na2SiO3, and thus the elution capabilities of the four modes increased evidently. The elution process of endosulfan in contaminated aged soil could be described by a4-parameter biphasic first-order reaction kinetic model, and showed obvious rapid and slow elution phases. The addition of Na2SiO3could increase the rate constants of rapid elution and slow elution, and decrease slow elution percent. Both of the elution percent and elution rate of β-endosulfanl were lower than those of a-endosulfan, which indicated that β-endosulfan in soil was difficult to be eluted. Compared with the other elution modes, endosulfan could be effectively and rapidly eluted by Tween80/SDS in the presence of Na2SiO3. and the elution percents of α-,β-endosulfan in contaminated aged soil reached the maximums (91.41%and74.01%) by parallel desorption for14and20h. respectively.2. A strain named as EB-4was isolated from the aged soil contaminated by endosulfan, and was identified as Ochrobactrum sp. by16S rDNA sequence analysis. The optimum degradation conditions of this strain to degrade endosulfan in three surfactant solutions were obtained in Eluent1(Tween80+Na2SiO3), Eluent2(Tween80/SDS) and Eluent3(Tween80/SDS+Na2SiO3), i.e., pH7.5～8.5.35℃, the microbial inoculation over10%, and the adding content of glucose1mg·L-1. The degradation process of endosulfan in those surfactant solutions could be described by the first-order reaction kinetic model, and the half-lives of a-endosulfan were3.83,5.29, and4.53d, and those of β-endosulfan were3.35,4.50. and3.79d, respectively. The degradation rate of this strain for β-endosulfan was faster than that for a-endosulfan.The experimental results showed that endosulfan in Eluate1, Eluate2and Eluate3could be completely degraded in12d for surfactant elution followed with microbial degradation. The degradation percents of endosulfan in soil were more than92%in15d for simultaneous elution and microbial degradation. The microbial degradation rate of endosulfan was lower than the elution rate from the contaminated soil, and the former was the rate determining step in the whole process.Ochrobactrum sp. can degrade endosulfan with two pathways, i.e.. hydrolysis and oxidation. Hydralysis is the main pathway, and the main metabolic product is endosulfan diol which can be degraded further by this strain, and thus it may not accumulate in soil and eluent. The main metabolite of oxidation is endosulfan sulfate which is difficult to be degraded further, and can be accumulated in soil and eluent.3. Nitrogen and fluorine doped Anatase TiO2could be synthesized by sol-gel method, and the average particle sizes were in the range of310.8-345.6nm characterized by X-ray diffraction and laser particle size analysis. The results showed that the two doped TiO2had photocatalytic activity for endosulfan in soil eluate under the irradiation of xenon lamp, and high photodegradation rates could be obtained under the conditions of strong acid and strong alkaline for surfactant elution followed with photocatalytic degradation. It was found that the optimum dosage of photocatalyst was400mg·L-1. and SDS in the eluent could promote and inhibit the photodegradation of endosulfan under the acid and alkaline conditions, resepctively. The photocatalytic degradation of endosulfan in soil eluate could be described by the first-order reaction kinetic model. When N-doped TiO2was used to be photocatalyst, the half-lives of a-endosulfan in Eluate1’, Eluate2’and Eluate3’were71.7,43.0and58.7min, and those of β-endosulfan were85.3,51.1,66.5min, respectively. When F-doped TiO2was used to be photocatalyst, the half-lives of a-endosulfan in Eluate1’, Eluate2’and Eluate3’were121.6,138.6and165.0min, and those of β-endosulfan were135.9,154.0and177.7min, respectively. It indicated that the photocatalytic activity of N-doped TiO2was higher than that of F-doped TiO2, and the photodegradation rate of β-endosulfan was lower than that of a-endosulfan.For simultaneous elution and photocatalytic degradation, the photodegradation percents of a-endosulfan with the elution of Eluent1-Eluent3using N-doped TiO2were76.47%,74.81%and79.14%at120h irradiation, while.that of β-endosulfan were57.79%,57.21%and58.52%, respectively. The photodegradation percents of a-endosulfan increased to75.65%,73.59%and78.18%using F-doped TiO2at120h irradiation, while that of β-endosulfan increased to57.12%,56.41%and57.36%, respectively. The results also showed that the photodegradation rate of endosulfan was faster than the elution rate from soil, and the surfactant elution was the rate determining step in the whole process. The photodegradation process could be well described by a four-parameter biphasic first-order reaction kinetic model, and showed obvious rapid and slow photodegradation phases, the latter was the rate determining step. The photodegradation rate of β-endosulfan in the contaminated aged soil was lower than that of a-endosulfan, because β-endosulfan would be more strongly adsorbed on soil colloids. The degradation percents of endosulfan in different eluents followed a decreasing order of Eluent3, Eluent1and Eluent2, and were the similar as the variations of the elution percents. Photooxidation is the primary pathway for the photocatalytic degradation of endosulfan, and the degradation product endosulfan sulfate could be detected both in soil and the supernatant, which could be degraded quickly by photooxidation. Endosulfan sulfate in soil could remain longer than in the supernatant, and needed14h to completely remove by photodegradation.4. With the soblization of surfactants, microbial-photocatalytic combination degradation could shorten the reaction time, and improve the degradation efficiency of endosulfan in the contaminated soil, and thus could effectively avoid the accumulation of endosulfan sulfate. Compared with the removal percents of endosulfan in soil by simultaneous elution and microbial degradation for3-9d followed with photodegradation, it could be found that the longer of the time of simultaneous elution and microbial degradation, the higher the removal percent of endosulfan by combination degradation. Among them, the removal percents of α-,β-endosulfan in contaminated aged soil spiked with20mg·kg-1endosulfan could reach96.57%and91.25%by N-doped TiO2photocatalytic degradation for120min after simultaneous elution using Eluent3and microbial degradation for9d, respectively, and the micariobial metabolite endosulfan sulfate could be also removed well, and thus may effectively remedy the endosulfan-contaminated soil.