Characteristics Of Pore Structure And Connectivity In Clayey Media And Its Water Distribution

Soil pollution is an important part of the ecological and environmental problems nowadays,which including industrial pollution,agricultural pollution and household waste pollution.Pollutants include heavy metals,pesticides,fertilizers and so on,most of which exist for a long time and are difficult to resolve.Dealing with soil pollution is often time-consuming and costly,which cause some places to be abandoned.One of the difficulties of repairing is the pollutants are concentrated in the microscopic pore space of soil,and the convection cannot directly affect the migration and the dissipation of pollutants.Clay layer,in particular,has been treated as an impermeable layer because gravity water is difficult to penetrate in the convection process,but it is often the place where pollutants accumulate the most.Understanding of the microphysical environment in clay is not only conducive to understanding the storage space and transport channels of pollutants,but also contribute to understanding the long-term slow transport behavior of pollutants.In this paper,several high-purity clay minerals and natural clays were taken as research objects,the latter were collected from Jianghan Plain.A series of experiments were carried out to study the pore structure which includes pore size distribution,morphology,connectivity and so on.Then,the migration or storage of different fluids in clay were studied basing on the experiment results.The characterization of pore connectivity and the diffusion behavior of gas in pore channels is studied with non-wetting gas as medium.The adsorption behavior and distribution of water in the pores were analyzed with wetting gas(water vapor).The main methods and results of this thesis are as follows:1.Combining mercury injection porosimeter(MIP),nitrogen physisorption,water vapor adsorption and small angle neutron scattering(SANS)experiments,the pore types of clay were identified and the pore structure was characterized by pore size distribution,permeability,and fractal characteristics.The dual pore system in the clay was identified and the controlling factors were analyzed.(1)The pores in clay can be divided into two parts:micron and nanometer pore systems.The micron pores are mainly ink-bottle shape,and most of the pore diameters are in the range of 0.1?1?m,and the pore volume mainly concentrates in 1?m pores.Nanopores are mainly slit type,the pore widths are mainly concentrated in 2?13nm,the pore volume is mainly composed of 10nm and 100nm pores.According to the average total pore volume,nanopores account for about 29%of total pore volume,which cannot be negligible in clays with porosity of more than 20%.This provides a physical spatial evidence for the storage of pollutants in nanoscale pores.(2)The pore structure of clay is controlled by external and internal cause.The external cause is the gravity compaction effect of the upper load on the arrangement of clay particles,which mainly acts on the micron pores.The total volume of the micron pores decreases by about 0.021 cm~3/g with the increase of 1m depth.The internal cause is mainly the clay mineral content in clays,which mainly acts on the nano pores.The total volume of the nano pores increases by about 1 mm~3/g with the increase of the clay mineral content by 1%.As a result,micron pores are mainly composed of interparticle pores and nano pores are mainly composed of clay mineral pores within particles.2.Based on the dual pore system in clay,a new fluid migration method is explored to represent the dual pore channels.The traditional homogeneous model cannot describe this medium space and fluid migration accurately.Therefore,in this thesis,basing on the gas diffusion experiment,non-wetting gas(oxygen)is used as fluid medium,and the traditional calculation method is extended to two-channel method.Then,assuming that micron pores correspond to interparticle pores and nano-pores correspond to intraparticle pores,the model parameters of inter or intra particles of clay are obtained by experimental method,and the gas diffusion coefficients of the two pore systems are obtained and used to represent the connectivity of pore channels.Moreover,the gas diffusion behavior is analyzed and evaluated under the condition of dual pore system in clay,which provides a new evaluation method the physical space influence on the fluid migration.(1)Interparticle channels are the dominant channels of gas diffusion,but intra-particle channels still work and become more and more important with the increase of total porosity.When the intergranular channels are determined,the increase of the total channel diffusion is mainly due to the increase of the porosity of the intergranular channels.In addition,when the diffusion is in a closed system,the inter-and intra-particle channels influence each other,and the dominant channels will suppress the diffusion of the other channels.(2)When porous media are strong heterogeneity and exist obvious dominant channels,most of gas will directly migrate through the dominant channels and no longer enter the inferior channels,which lead to the invalidation of some pores in diffusion.In the experiment set up in this study,the effective porosities of inter-and intra-particles are linearly correlated with the total porosity.In addition,when interparticle porosity is 0.4,the intraparticle channels has no effect on the diffusion unless its porosity is more than0.2.3.Water is the most important contaminant carrier in clay,and also an unavoidable factor in analyzing the pore structure in clay.At present,most of the descriptions of soil water are usually simplified to a unified water type or divided into capillary water and bound water,which is not precise enough in the microscopic pore space of clay.In order to emphasize the adsorption mechanism of bound water,this study calls it adsorbed water.Basing on the pore structure analysis of clay,a slit pore model is established to describe the growth of capillary water and adsorbed water during the dry to wet process in clay.The adsorbed water is further divided into adsorbed water film and interlayer water of clay minerals.Firstly,the basic calculation formulas of three water forms are defined,and then the relationship between water film thickness and relative humidity is obtained by the stability analysis of interfacial forces on the surface of the water film.Finally,the distributions of water contents of various water forms are obtained,which provides a basic evidence for evaluating the storage and migration of pollutants in micro-pores of clay.(1)The interlayer water in clay minerals creates new pore spaces that cannot be unexplored by non-wetting fluids.The maximum interlayer water content is 0.17 g/g in high purity montmorillonite.The adsorbed water content,which is calculated by adding interlayer water content and capillary water content,is 86%of total water content in montmorillonite while the total water content is 0.21 g/g.The proportion of adsorbed water in kaolinite is the lowest,but still reaches 11%of total water content.(2)Based on pore structure of clay,the water film is mainly controlled by specific surface area and water film thickness;the capillary water is mainly controlled by pore size distribution.Basing on the slit-pore model,theoretically,the minimum thickness of water film is 0.23nm and the maximum thickness is up to 1/3 of the pore width.