A Study For Contaminant Extraction In Polluted Soil Using PVD-enhanced System

Prefabricated vertical drains (PVD) were introduced in a scheme for the expedient flushing of contaminated fine-grained soils. The PVD-enhanced system works by shortening the flow path to accelerate the remediation process; however, the current analytical solution that considers the infinite extraction boundary is not suitable to describe the close distance between the injection PVDs and the extraction PVDs.A Neumann boundary is proposed as an extraction boundary that can reflect the characteristics of the PVD-enhanced system. A simplified planar two-dimensional model for contaminant extraction is proposed for the case that the PVDs are arranged in a grid pattern; another simplified axisymmetric model is proposed for the case that the PVDs are arranged in an axisymmetric pattern. Considering the complicated conditions around the PVDs, this paper presented a simplified analytical model using drain wall considering the smear zones, and a soil remediation model for soil flushing through clogged geotextiles with a filter cake. All the above models are based on the assumption of the linear equilibrium desorption between the soil samples and the contaminants, however, the trailing phenomena discovered in the soil column tests for soil flushing demonstrated the nonequilibrium desorption characteristics between some soil samples and contaminants. This work proposed an analytical model considering the nonequilibrium desorption charateristics between the soil samples and the contaiminants, and based on this theory, the mechanisms and the charateristics of nonequilibrium desorption are discussed. Parametric studies are performed to investigate the effect of influence factors on the efficiency of the soil remediation. The main conclusions are as follows:(1) A simplified planar two-dimensional model for contaminant extraction is proposed for the case that the PVDs are arranged in a grid pattern, and the analytical solution is obtained. The Neumann boundary in this model can reflect the characteristics of the PVD-enhanced system that the system works by shortening the flow path to accelerate the remediation process; the flux-type injection boundary leads to conservative results. The model accounts for the initial condition that is exponentially distributed along the depth. Moreover, the analytical solution based on the simplified model of drain walls can simulate the concentration for most locations in the contaminant zone for any particular time using PVD-enhanced system.(2) It is indicated that increasing the injection/extraction rate can improve the efficiency of the soil remediation, and the time needed to clean up the contaminated soil approximately decreases linearly with the increase of injection rate; it is also showed that shortening the distance between the injection and extraction PVDs can expedite the remediation process. In the preliminary design of the PVD-enhanced system, we should take into account the scale of the contaminanted field and the technical capacity, and optimize the key parameters of injection/extraction rate and the distance between the injection and extraction PVDs for cost saving.(3) A simplified axisymmetric model is proposed for the case that the PVDs are arranged in an axisymmetric pattern, and the solution is obtained using the semi-analytical method. The initial condition that is exponentially distributed along the depth is taken into account. It is suggested that the semi-analytical solution based on the simplified drain wall model can adequately simulate the concentration for any location in the contaminant zone for any particular time, even though only3extraction PVDs are distributed around the injection PVD.(4) A simplified analytical model using drain wall considering the smear zones, and a soil remediation model for soil flushing through a clogged geotextiles with a filter cake are proposed. The analytical solutions are obtained. Based on the simple analytical solutions, the effect of the complicated conditions around the PVDs are discussed.(5) For the trailing phenomena discovered in the soil column tests for soil flushing, an analytical model considering the nonequilibrium desorption charateristics between the soil samples and the contaminants is proposed. The model is generic which can both well reflect the equilibrium or nonequilibrium desorption characteristics between different soil samples and contaminants. Compared with the equilibrium desorption theory, the nonequilibrium desorption theory proposed in this work is more reasonable, which can better reflect the mechanisms and the charateristics of soil desorption, better predict the concentration for any location in the contaminant zone for any particular time and evaluate the experimental results.