Mechanism Of Montmorillonite-mediated Biodegradation Of Phenanthrene By Sphingomonas Sp.GY2B

Organic pollutants such as PAHs accumulate in the natural environment in various ways,pose a great threat to human health and the ecological environment.The slow and continuous coexistence between active degrading bacteria and minerals in soils is one of the most important environmental and geochemical processes in the epigenetic environment,which plays an important regulatory role in the remediation of PAHs in soil.In this paper,phenanthrene as a model compound in PAHs was studied.Sphingomonas sp.GY2B,a highly efficient degrading bacterium selected from PAHs contaminated soil was combined with most common naturally occurring clay-montmorillonite（Mt）to form the clay-microbe complex,which greatly improved the degradation efficiency of phenanthrene.Meanwhile,in order to further explore the interaction mechanism among phenanthrene,degrading bacteria and Mt,this complex system was divided into the adsorption of microorganisms on the mineral surface,the influence of minerals on the growth and activity of microorganisms,and the influence of microorganisms on the physicochemical properties and structure of minerals.Based on the in-depth analysis of adsorption,degradation,mineral dissolution,the interfacial reaction mechanism between clay and microbe in the clay-microorganism complex,the degradation mechanism of phenanthrene,and the influence mechanism of bacteria on the physical and chemical properties of minerals were systematically elaborated,which provided theoretical support for the ecological remediation of PAHs and the environmental application of mineral materials.The above research has achieved the following results:（1）The effects of bacteria on the structure,surface morphology and physicochemical properties of minerals（including montmorillonite and modified montmorillonite）were investigated.Firstly,the degradation of phenanthrene by microbial-mineral complex was carried out.Within 48 h,the complex could degrade more than 99%phenanthrene at the initial concentration of 100 mg L^-1in comparison with the degradation of free GY2B（about 80%）.The degradation of phenanthrene after bacteria adsorption onto montmorillonite had obvious advantages,showed that the addition of clay minerals promoted the proliferation of bacteria as well as the degradation of phenanthrene.Then,various characterization methods were used to evaluate the changes of the structure and morphology of montmorillonite and modified montmorillonite by bacteria,especially the determination of the dissolution concentration of the main elements（Si and Al）in the solution,revealing the preferential dissolution of Si from the tetrahedral sheet in the dissolution process.Finally,the dissolution mechanism of minerals during the mediated-degradation of phenanthrene was analyzed,that is,the effect of pH effect and ligand promotion.（2）The interaction mechanism between montmorillonite and Sphingomonas sp.GY2B was explored.In this study,thermodynamics analysis combined with the extended DLVO theory were employed to investigate the adhesion of Sphingomonas sp.GY2B on Mt.The experimental results could be described well by pseudo-second-order（R²=0.997）and Langmuir model（R²=0.995）.The thermodynamics analysis revealed the physical nature of bacterial adhesion onto Mt,which was confirmed by the extended DLVO theory.The related surface properties（Zeta potential,hydrodynamic diameter and hydrophobicity）at different ionic strength were determined and the interaction energy between Mt and GY2B were also calculated.At low ionic strength（≤20 m M）,GY2B adhesion onto Mt was primarily driven by long-range DLVO forces（e.g.electrostatic repulsion）,while short-range（separation distance<5 nm）Van der Waals and hydrophobic interactions played more important roles in the bacterial adhesion at higher ionic strength（50-100 m M）.（3）In the low organic matter aqueous environment,the adsorption characteristics and adsorption mechanism of low concentration(1 mg L^-1 and below)phenanthrene in aqueous solution on two typical clay minerals,montmorillonite and kaolinite,were systematically studied.The effects of different environmental factors such as reaction time,solution pH,mineral dosage,ionic strength and temperature on the adsorption of phenanthrene on minerals were investigated.It is found that the phenanthrene can quickly reach the adsorption equilibrium within 60 min at the beginning of the reaction,and the pseudo-second-order kinetic model can well fit the experimental data.The effect of solution pH on the phenanthrene adsorption process was not obvious in the experiment,which was caused by the inability of phenanthrene to ionize within the selected pH range.The addition of ionic strength can greatly promote the adsorption of phenanthrene on the mineral surface due to salting out effect.The adsorption process of minerals on phenanthrene was an exothermic process,and low temperature was more conducive to the adsorption of phenanthrene.The adsorption experiments of saturated cation montmorillonite showed that the adsorption of phenanthrene on the mineral surface was driven by hydrophobicity and also influenced by the specific surface area of the mineral and the type of interlayer cation.The adsorption capacity of montmorillonite on phenanthrene was obviously superior to that of kaolinite.One reason was the difference in specific surface area,and the other was determined by cation exchange capacity.