| With rapid economic growth, urbanization and steady improvement of peoples'consumption level, the waste disposal and pollution prevention have already become pressing environmental protection issues in the urban areas in our country. There is an obviously increase in the organism proportion in composition of urban consumer waste. Due to simple technology, easy operation, low investment and construction cost, landfill is still the major method used in urban garbage treatment, and the harmless treatment level is extremely low. Improper treatment of garbage has resulted in a series of impacts, including the soil damage and pollution, groundwater resources pollution, aggravation of environmental pollution as well as severe harm to urban environment and living conditions of residents. It has severely restricted the social, economic and ecological sustainable development of the city, affected the construction of harmonious society and aroused great concerns of relevant departments for a long time. Many domestic and overseas scholars have analyzed the organism composition in leachate, and the result shows that the composition of organic pollutants is complicated:mainly are alkenes and aromatic hydrocarbon, and many compositions are synthetic organic compounds which never occurred in nature in the past. Considerable compounds are carcinogen and toxic pollutants. Therefore, to control the pollution caused by organic pollutants is a key step in landfill leachate treatment. In the landfill engineering, the barrier layer of cohesive soil is the natural barrier for garbage treatment, and the nature, density and thickness of cohesive soil layer directly decides whether the landfill can pollute groundwater and the degree of its pollution. Tightness and permeability must be considered in landfill construction, and the selection for filling layer is the key aspect for solving the above two issues. Therefore, to research on purification capability of different cohesive soils on organic pollutant compositions of garbage leachate is an important link for landfill site selection and groundwater pollution prevention.Naphthalene, Phenanthrene and Fluoranthene belong to the persistent organic pollutant—PAHs, and their content in the leachate of municipal waste is high. Among them, naphthalene is 2-rings, Phenanthrene 3-rings and Fluoranthene 4-rings, and the properties of PAHs (especially the number of ring) have a great influence on its migration in soil. By selecting these three organic pollutants as research objects, and according to the adsorption oscillation experiment and soil-column vertical leaching simulation experiment, it quantitatively analyzes the vertical migration rule of those objects in the barrier layer of cohesive soil with different densities and determines retention capability of cohesive soil in different densities on PAHs with different rings, providing basis for the design of seepage-proof bedding layer of the landfill.Based on the adsorption dynamics experiment, isothermal adsorption experiment and adsorption influencing factor experiment, the thesis analyzes the adsorbability of landfill barrier cohesive soil layer on PAHs. Based on the results of those experiments, it discusses the adsorption mechanism and the function mechanism of influencing factor.1. Silty clay mainly uses the surface adsorption for adsorbing the PAHs, and due to low content of organic compounds in the soil samples, the partition function plays a supporting role rather than a leading one. According to adsorption dynamics experiment, it needs 1h for the silty clay's adsorption on the three organic compounds—naphthalene, phenanthrene and fluoranthene, to achieve the equilibrium, and obvious stages can be seen in the adsorption. There are great differences in the three adsorption rates:39.76% for naphthalene,78.09% for phenanthrene and 97.60% for fluoranthene. There is a slight difference between the adsorption rate of the three PAHs monomers and the mixed solution, and the interaction of the three has a small influence on the adsorbability of cohesive soil. The isothermal adsorption curve of naphthalene is more consistent with Langmuir equation model, while the isothermal adsorption curves of phenanthrene and fluoranthene better fit the Freundlich equation model. In this equilibrium system, the soil-water partition coefficients of naphthalene, phenanthrene and fluoranthene are 0.72,4.76 and 10.56 respectively.2. In the equilibrium system of this experiment, temperature and pH value have a certain influence on adsorbability of cohesive soil, and the adsorption on the three substances are different if the temperature changes. As the volatility of naphthalene is largely affected by temperature, the adsorption rate of naphthalene increases when the temperature increases. Adsorption is an exothermic process which can largely affect the adsorption of phenanthrene. The adsorption of phenanthrene decreases as the temperature increases. As the adsorption rate of fluoranthene is high and the external environment's influence on it is small, the temperature's influence on the adsorption of fluoranthene is small. When approaching to 7.8, the background value of pH value, the curve fluctuation is relatively large. When it is at the background value, PAHs has the minimum adsorption amount. It can enhance the adsorption amount of cohesive soil on PAHs by properly adding acid or alkali, and the influence on adsorption performance is more obvious by adding alkali.3. With the increase of mineral composition of montmorillonite in cohesive soil for test, there is an increase in the adsorption rates of naphthalene, phenanthrene and fluoranthene, among which the influence on fluoranthene is the biggest. If adding 30% of montmorillonite, the adsorption rate of fluoranthene reaches 100%. Moreover, the higher of the pure cohesive soil mineral content in the cohesive layer is, the higher adsorption rate of the PAHs is; within the same time period for adsorption, the adding of humic acid has greatly increased the adsorption of cohesive soil on PAHs. And it has achieved the best adsorption effect when the mass ratio of humic acid in the mixed soil is 20%. By comprehensive consideration, when humic acid in the soil reaches this ratio, it reaches the best for its adsorption capability on PAHs; the thinner particles of soil and larger specific surface area, will result in a stronger surface effect. The maximum adsorption amount of PAHs in the three soil samples are respectively:clay>silty clay >fine sand. The clay particle composition in the soil sample plays an important role in the adsorption, and the clay has larger specific surface area compared with other components, more surface adsorption points, and shows a superior adsorption performance.4. In the leaching experiment of PAHs, the total amount of sewage interception for naphthalene is 5.81mg with a purification rate of 29.074%; for phenanthrene, it is 19.98mg with a purification rate of 99.894%; for fluoranthene, it is 19.99mg with a purification rate of 99.988%. The unit sewage interception amounts for the three are 3.79μg/g,13.06μg/g and 13.06μg/g respectively. It shows that the silty clay has strong sewage interception capability on phenanthrene and fluoranthene. The vertical migration of PAHs in the silty clay simulation column is a process of chemical non-equilibrium, and it can better fit the break-through curve of PAHs by applying the two-point models in the CXTFTIT software, whereas it is due to the insufficiency of the porous medium and the dispersion coefficient precision of flow field nature, limitation of experimental data as well as the not-so-high discrete fitting correlation coefficient in the soil-column characterization achieved by the tracer experiment.5. Under the different compaction conditions, mechanical properties of silty clay in soil columns have also changed a lot. The test result shows that with the increase of compaction times and the compaction function on soil, the density of clay increases subsequently and its saturation also increases, showing positive correlation for the growth situation; with the decrease of void ratio, the permeability coefficient continuously decreases; it shows a negative correlation:the compressive modulus of soil first increases and then decreases while the compression coefficient shows an reverse trend.6. In the experiment of barrier action of silty clays with different densities on PAHs, the effective barrier thickness for phenanthrene and fluoranthene is 10cm. The silty clay can realize effective barrier on phenanthrene and fluoranthene within 10cm thickness, whereas the sewage interception rate of naphthalene in the silty clay is relatively small, and 10cm thickness can not form an effective barrier on its migration in the silty clay. There is a big difference in the sewage interception amount of barrier layer of cohesive soil with different densities on naphthalene, phenanthrene and fluoranthene. For naphthalene, it changes a lot in its sewage interception capacity and there is a trend of increase first and decrease next with increase to 9.7μg/g at the 38th compaction from 4.7μg/g at the 11th compaction. With the increase of compaction strength, the sewage interception of naphthalene gradually decreases to 8.2μg/g at the 50th compaction. Under the condition of the same soil quality, it has small compaction amount and large thickness for barrier layer of soil, but has small density. Therefore, it has a relatively large seepage discharge and organic pollutants will continuously infiltrate via soil voids. With the continuous increase in compaction amount and density of barrier soil layer, it can effectively prevent the infiltration of pollutants. When the compaction amount increases to a certain critical value, the density of the cohesive soil will increase. Moreover, the adsorption amount of barrier layer will reach some boundary value, and the adsorption amount decreases. There is a subtle change in the adsorption amount of phenanthrene and fluoranthene as the compaction strength changes. The sewage interception capacity of phenanthrene and fluoranthene increases as the compaction strength changes, but the variation is not so obvious. As the sewage interception amount of both phenanthrene and fluoranthene is relatively large and nearly reaches the saturation value. The influence of density can not be completely showed in this experiment.By taking a comprehensive consideration of temperature, pH value, organic matter content, particle size, clay and other factors in the barrier cohesive soil layer of the landfill, it will help to improve the purification performance on the organic pollutants. Through finding out the influences and changes of the density of the cohesive soil on organic pollutants, it can define the best cohesive soil density which has the best barrier performance on organic pollutants. With quantification on effective barrier thickness of organic pollutants, it can optimize the design of the barrier layer of the landfill. |
