Similitude For Centrifuge Modelling Of Heavy Metal Migration In Clay Barrier And Method For Evaluating Breakthrough Time

Analytical solutions or numerical methods were always used in predicting and evaluating the long-term performance of barrier systems in solid waste landfill and pollution control engineering of contaminated site. The predictions of breakthrough time lack of validation of long-term field data or model test data. Based on clay barrier and heavy metal Pb2+ as the object of study, a series of the batch test, diffusion test, 1g column test and centrifuge test have been adopted to systematically study centrifuge modelling technology of pollutant migration. A method for evaluating breakthrough time based on centrifuge model test was put forward. A 21-year-long process of Pb2+ transport in a kaolin liner was successfully simulated for the first time in a centrifuge model test in which a centrifuge was operated continuously for 72 hours. A series of significant conclusions were drawn accordingly.(1) A set of test devices for centrifuge modeling pollutants transport process in clay barrier were invented. A Modified Mariotte bottles have been employed to keep the constant water head in centrifuge. And the outflow collection and monitoring devices can monitor the volume of the outflow and the concentration of the outflow solution that accumulated in the device under hypergravity conditions.(2) Seepage force and soil pressure in models have been analyzed considering acceleration changes with the depth of models. The results showed that the stress error between model and prototype was negligible as the height of model was less than 90cm for the ZJU-400g.t centrifuge. The effective centrifuge radius of model was determined. The maximum consolidation stress for model preparation was 400 kPa so the permeability coefficient of the model can be less than 10-7cm/s. And model under in centrifuge was over-consolidated, that can eliminate the consolidation of model by centrifugal force and the influence on the pollutants migration process.(3) Diffusion tests, 1g column tests and centrifuge column tests with various hydraulic heads have been conducted. The relation between the hydrodynamic dispersion coefficient and the seepage velocity was established, and the similitude of dispersion was discussed. It was found that the critical Peclet number (Pe=1) previously-reported can be used for judging similitude of dispersion in Kaolin clay. When Pe was defined based on mean particle size, the critical Peclet number was much less than 1. When the centrifuge test was used for modelling non-adsorbable pollutant transport in a 2-m-thick low-permeability clay liner prototype, the error in predicting breakthrough time was related to waterhead difference and centrifugal acceleration. When the centrifugal acceleration was less than 100g, the errors for all possible waterhead differences (0-40m) in landfill were less than 24%.(4) The results of the diffusion test, 1g column test, and the 24h centrifuge model tests were compared. Influences of ratio of soil and water (S/W) and velocity on adsorption were discussed. It showed that the kaolin’s adsorption in soil column with high S/W was higher than batch test with low S/W; the retardation factor decreases from 12.5 to 8 as the velocity increases from 0 to 5.67 × 10-7m/s, the retardation factor keep constant as the velocity increases from 5.67 × 10-7m/s to 14.8 × 10-7m/s, the ralation between the retardation factor and the velocity was established. Adsorption process was not completely identical in the model and prototype, as Pb2+ transports in low permeability kaolin clay liner simulated by centrifugal model test. The breakthrough time predicted by the model test was smaller than the prototype breakthrough time, and the error can be modify with Rd-vs.(5) A method of evaluating barrier’s breakthrough time based on accelerated physical model experiment was proposed for the adsorptive pollutants. The one-dimensional advection-diffusion model which can describe the clay’s adsorption behavior was chosen, the suitable boundary conditions were determined by the actual situation; diffusion coeficient was determined by diffusion test, inert ion soil column tests was used to determine laboratory dispersity, and the prototype dispersity was determine by empirical formula, a batch of short column breakthrough test with different source concentration were used to determine adsorption model and parameters, Finally typical centrifugal model test was carried out for validation. A 72-hour centrifuge test simulating 21-year-long process of Pb2+ transport in a model kaolin liner under a high water head was conducted to clarify the method.(6) The influence of source concentration, adsorption isotherm model and waterhead on the barrier performance were discussed based on the evaluation method. The results showed that the absolute breakthrough time of three adsorption model all increased as the source concentrations decreased, and the amplitude depend on the adsorption isotherm model:As the concentration decreased from 1000mg/L to 10mg/L, the breakthrough time increased by 0.6 yr (Freundlich model),5.3 yr (Linear model),13.3 yr (Langmuir model); the adsorption isotherm model had significant effect on containment preformance:the absolute breakthrough time of the convex type (Langmuir) adsorption model was 2 times of that of linear model, was 12.8 times of that of Freundlich model; An increase in waterhead from 0.3m to 10m results in an increase by 3 times in the breakthrough time considering that the decrease of velocity can increase the adsorption.(7) Analytical solution for one-dimensional diffusion of an organic solute in the three-layered composite barrier was obtained. Parametric analyses were carried out to investigate the. effect of primary factors on the barrier performance based on the analytical solution. The following conclusions and suggestions are drawn:the containment performance of the composite barrier depends much on the partition coefficient of the HDPE geomembrane over the target pollutant. The HDPE geomembrane exhibits an excellent resistance to the hydrophilic organics, which have a low partition coefficient. However, the hydrophobic organics, which have a high partition coefficient, can diffuse more easily through the HDPE geomembrane. An increase in the retardation factor from 3.3 to 30 leads to an increase by 7.6 times in the breakthrough time. Increasing the wall thickness not only delays the occurrence of the stable mass flux, but also significantly reduces its magnitude. An increase in wall thickness from 0.6m to 1.2m results in an increase by 2.8 times in the breakthrough time.