【作者】 陆海军；

【导师】 栾茂田；

【作者基本信息】 大连理工大学 ， 岩土工程， 2009， 博士

【摘要】 垃圾填埋场对环境的污染是陆地填埋法面临的主要环境岩土工程问题。为控制渗滤液过量排放而引起环境污染,现代卫生填埋场包含有防渗结构与封顶结构。压实黏土衬里（CCL:Compacted Clay Liner）广泛应用于填埋场防渗结构中。黏土可吸附多种污染物,但吸附能力较弱,提高CCL吸附能力对于控制填埋场对环境污染具有重要意义。在黏土中分别添加活性炭（GAC:Granular Activated Carbon）、酸活化膨润土、石灰等对其进行改良,以提高CCL的吸附及承载能力。采用吸附试验、土柱试验等方法,研究改良衬里的吸附特性,并确定了Cr（Ⅵ）在衬里中运移的弥散系数。采用土工试验对改良黏土作为填埋场衬里建造材料的可行性进行了研究。基于温度变化下污染物运移模型,对Cr（Ⅵ）在改良衬里中的运移过程进行数值分析。另外,对压实黏土封顶层与蒸发传输（ET:Evapotranspiration）封顶层的水分平衡进行了数值分析。基于试验与理论分析,取得了以下研究成果。1.吸附动力学研究结果表明,黏土对Cr（Ⅵ）、Zn（Ⅱ）的吸附可以通过伪二阶动力模式来描述;黏土对Cr（Ⅵ）、Zn（Ⅱ）离子吸附的活化能为22.7kJ/mol、26.88kJ/mol,说明黏土对Cr（Ⅵ）、Zn（Ⅱ）离子的吸附属于化学吸附;热力学参数说明黏土对重金属离子的吸附是吸热反应,溶液温度的升高能促进黏土对重金属离子的去除率。2.吸附试验结果表明,改良黏土试样对Cr（Ⅵ）的吸附均符合Langmuir等温吸附方程,且吸附强度显著提高。在等温条件下,在双对数坐标系中,Langmuir吸附参数（q_m、b）随着土固体颗粒浓度的增加而线性减小,当土固体颗粒浓度增至临界值时（如:200g/L）,可达到稳定状态。在非等温条件下,吸附参数（q_m、b）均随温度升高而线形增大。3.土工试验结果表明,在重型击实及特定含水率下,黏土、经GAC或酸活化膨润土改良的黏土均能满足体缩率小于4%、渗透系数小于1×10^-7cm/s、无侧限抗压强度大于200kPa的工程要求,说明这些土料可以作为填埋场衬里的建造材料。4.由上层含石灰的黏土和下层含GAC或酸活化膨润土的黏土构成的双层衬里不仅能满足低渗透系数的工程要求,且明显的提高了衬里的无侧限抗压强度,增强了衬里的承载能力,进而提高了填埋场的填埋储量。5.以土柱试验所得到的穿透曲线拟合数值计算结果,分别确定Cr（Ⅵ）在单层与双层衬里中的弥散系数。在单层衬里中,弥散系数（D）分别为:含10%石灰的黏土为6.4×10^-8m²/s;黏土为3.0×10^-10m²/s;含3%或6%活化膨润土的黏土分别为2.2×10^-10m²/s与6.8×10^-10m²/s;含3%或6%GAC的黏土分别为2.5×10^-10m²/s与8.3×10^-10m²2/s。在双层衬里中,上层土为含10%石灰的黏土,其D值为4.4×10^-8-5.2×10^-8m²/s;下层土分别为:含3%与6%活化膨润土的黏土,其D值分别为6.1×10^-10m²/s与5.4×10^-10m²/s;含3%与6%GAC的黏土,其D值分别为6.5×10^-10m²/s与7.0×10^-10m²/s。6.考虑污染物在多孔介质中的对流、弥散、地球化学反应以及温度对吸附量影响条件下,建立了污染物在饱和双层结构体系中运移的数学模型,采用有限差分法对模型进行数值求解,并对Cr（Ⅵ）的运移过程进行数值分析。计算结果表明,当温度升高时,对于不同类型的双层衬里,Cr（Ⅵ）穿透速度由大到小的顺序依次为双层衬里1（上层为含10%石灰的黏土、下层为黏土）>双层衬里2（上层为含10%石灰的黏土、下层为含3%GAC的黏土）>双层衬里3（上层为含10%石灰的黏土、下层为含3%酸活性膨润土的黏土）,表明采用GAC、酸活化膨润土改良的CCL可有效延迟Cr（Ⅵ）的运移。探讨了填埋场水位、压实土层的渗透系数和弥散系数对Cr（Ⅵ）运移的影响,计算结果表明,当提高填埋场水位、增大渗透系数与弥散系数时,Cr（Ⅵ）在双层衬里中的运移速度明显加快,为Cr（Ⅵ）运移过程的控制因素。同时分析了土的比热和热传导系数对Cr（Ⅵ）在双层衬里中运移的影响,计算结果表明,土比热和热传导系数变化对Cr（Ⅵ）在双层衬里中运移影响较小,可不考虑其影响。7.以大连市1976全年的实际降水与蒸发蒸腾强度为边界条件,采用数值计算对比分析压实黏土封顶层与ET封顶层中水分运移特性,探讨了降水强度、降水持续时间、土层初始含水率与植物生长土层厚度等因素对封顶系统中水分运移的影响。数值计算结果表明,临近地表的土层,其含水率受降水、蒸发的影响显著,且随着深度的增加出现明显的“峰值滞后”现象;对于压实黏土封顶层,因渗透系数低,其含水率受降水、蒸发影响较小;对于ET封顶层,降水可有效补给整个土层,而蒸发时,可有效排出土层中所储存的水,因此,可避免土层中水分过量蒸发而引起开裂,并可防止水分渗入填埋场,具有水分平衡自调节功能,是一种很有发展潜力的填埋场最终封顶层。更多还原

【Abstract】 The main environmental geotechnical problem of landfill is the pollution and destruction on environments induced by the municipal solid wastes. In order to control leachate leakage and transport, landfill liners and covers are commonly built in modern landfills. The compacted clay liners （CCL） are widely used in impervious structure of landfill. The clay is used as a retention medium of pollutants present in the leachate. However, the sorption capacity of pollutants onto clay is weak. Hence, there is a crucial need to improve sorption capacity of CCL. A method to amend the liners with granuar activated carbon （GAC）, bentonite activated by acid and lime capable of strongly sorbing pollutants is performed in the study. In order to improve the bearing capacity of liners, a method of adding lime into liner-soil materials is proposed. The adsorption property of improved CCL is observed by a series of batch tests. The pollutants transport through improved CCL is investigated by column tests. The diffusion coefficients of pollutants are determined according to data from column tests. The geotechnical tests are carried out to test the feasibility of improved clay used as liner-soil materials. A dynamic model is developed for simulating Cr（Ⅵ） transport in improved CCL. The moisture balance of compacted clay and evapotranspiration （ET） cover are analyzed by numerical simulation. On the basis of tests and theoretical analysis, the main conclusion is as follows:1. The pseudo-second order kinetic model is the best choice among all the kinetic models to describe the adsorption of Cr（Ⅵ） and Zn（Ⅱ） onto the clay. The activation energy values for Cr（Ⅵ） and Zn（Ⅱ） were found to be 22.7 kJ/mol and 26.88 kJ/mol. The value of activation energy suggests that the adsorption process might be a by chemical adsorption. The thermodynamic parameters values indicate that the adsorption process is an endothermic in nature and the rise in temperature favors adsorption.2. The results from batch tests show that the Cr（Ⅵ） sorption to improved clay was nonlinear and characterized by a Langmuir isotherm model. As the soil-solids concentration increases the Langmuir isotherm parameters （q_m,b） first decreases logarithmically, but then stabilizes when the soil-solids concentration exceeds a critical value （e.g.; 200g/L）; The isotherm parameters also increase linearly with increasing temperature.3. The results of geotechnical tests show that the compacted clay and clay containing GAC or bentonite activated will exhibit volumetric shrinkage≤4%, hydraulic conductivity ≤1×10^-7cm/s and unconfined compressive strength≥200kPa. These data indicate that the above soils can be used as liner-soil materials.4. A two-layer liner is designed; the upper soil layer is composed of 90% clay and 10% lime, and the lower soil layer is composed of 97% or 94% clay and 3% or 6% adsorbents （GAC or bentonite activated）. The two-layer liner can not only meet requirement of hydraulic conductivity, but enhance its bearing capacity and adsorption capacity to pollutants.5. The diffusion coefficients of Cr（Ⅵ） for liner-soil materials were back-calculated by a one-dimensional numerical transport program. The diffusion coefficients of one-layer liners were as follows: for 90% clay plus 10% lime, 6.4×10^-8m²/s; for clay, 3.0×10^-10m²/s; for 97% clay plus 3% bentonite activated, 2.2×10^-10m²/s; for 94% clay plus 6% bentonite activated, 6.8×10^-10m²/s; for 97% clay plus 3% GAC, 2.5×10^-10m²/s; for 94% clay plus 6% GAC, 8.3×10^-10m²/s. The diffusion coefficients of two-layer liners were as follows: for clay containing 10% lime of upper soil layer, 4.4×10^-8-5.2×10^-8m²/s; for clay containing 3% or 6% bentonite activated of lower soil layer, 6.1×10^-10m²/s or 5.4×10^-10m²/s; for clay containing 3% or 6% GAC of lower soil layer, 6.5×10^-10m²/s or 7.0×10^-10m²/s.6. On the basis of considering flow, diffusion, geochemical reaction and the effect of temperature on adsorption, a pollutants transport model is presented. Solutions to the model were accomplished by finite difference, which predicted Cr（Ⅵ） transport through two-layer liners systems. Results indicate that liner containing activated bentonite exhibited the highest retard capacity, followed by liner containing GAC and CCL. GAC and bentonite activated are two potential materials for their use as sorptive amendments for sorbing heavy metals in CCL. As leachate head, hydraulic conductivity, and dispersion coefficients increase, the Cr（Ⅵ） transport significantly speed up. Hence, they are main factors that influence the pollutants transport. The effect of soil specific heat and thermal conductivity to Cr（Ⅵ） transport is not obvious.7. Volumetric water content of compacted clay and ET cover is simulated under data of precipitation and evaporation of Dalian in 1976. Parametric analyses are conducted to simulate variations in precipitation rate, precipitation event duration, initial volumetric water content and top soil layer thickness. Results show that the moisture content near the surface is highly sensitive to climate loading; and the effect is significantly attenuated and time lagged along with increasing depth. The compacted clay cover has low penetrability; so it can not get effective moisture charge. ET cover acts as a Veservoir that stores moisture during precipitation events and subsequently returns it to atmosphere as evapotranspiration. ET cover prevents desiccation cracks and water leakage. Hence, it has great potentialities used in construction of landfill covers.更多还原