The low-viscosity stabilizer, colloidal silica, is extensively used as a grouting material in the construction of grout curtains. It has low viscosity and is non-toxic, which is suitable for injection to stabilize fine-grained soils. It is also applied as a stabilizer in the in-situ treatment of hazardous waste. Once the colloidal silica solution is injected into contaminated soil, it moves through the pores inside the soil matrix, initiating the stabilization process. The viscosity of the colloidal silica mixture increases while it moves until solidifications. This process is called gelation and results in the creation of a gel barrier around contaminated soil particles, causing a substantial reduction of fluid flow in the soil, this will minimize the movement of water and hence movement of contaminants through the gel mass.
In this research, the stabilization process of chromium contaminated soils using colloidal silica were investigated. The transport of colloidal silica during injection was simulated at the microscopic level to further understand the gelation process. Diffusion of chromium through colloidal silica gel was modeled to evaluate the effectiveness of the technology. A new optical method to estimate the diffusion coefficient of chromium in gel was purposed. The diffusion coefficient obtained using the above optical method was used to evaluate the long term effectiveness of colloidal silica grouting technology.
The movement of colloidal silica during the injection was modeled using the change in gel viscosity with time. The simulation showed that during the grouting process, solidification starts at the soil surface and expands to fill the void space within 1.2 hours. The different soil geometries resulted in the different velocity contours and different colloidal silica solidification patterns. The greater the ellipsoid axial ratio of soil resulted in faster solidification.
The measured diffusion coefficients of chromium in the colloidal silica gel, NYACOL DP5110® from Eka Chemical Inc., ranges from 1.76 to 8.48 x 10-10 m2/sec depending mainly on the concentration of silica in the gel and initial concentration of chromium. Higher silica concentrations yielded greater diffusion coefficients due to the obstruction to the free movement of chromium. The adsorption isotherm of chromate to colloidal silica gel was found to be linear at pH 7; partition coefficient was calculated to be 0.549 liter/gm. Mass balance calculations were performed to evaluate the accuracy of the proposed method and the error was less than 6.5%. Therefore, the optical method using digital imaging is an effective and reliable technique for measuring the diffusion coefficient of metal contaminated colloidal silica gel.
Using the measured diffusion and partition coefficients, a simulation was performed for worse case scenario, where chromium is continuously dissolved from soil into the water interface between soil and gel, and moves to groundwater without obstruction from soil particles. The thickness of the gel barrier used in this simulation was 5 cm (Heisher, 1997). The solubility of chromium in water was 109 mg/liter (Rock et al, 2001). The results showed that it would take approximately eight days before the groundwater would exceed the USEPA standard for chromium (0.1 mg/liter).
Based on these initial test results, the use of NYACOL DP5110® to treat chromium contaminated soil appears to be ineffective due to high diffusion. The diffusion of contaminant in gel will be a major concern when applying this technology. Further research of more realistic simulation of diffusion and refined gel formulation with capacity to convert the chromium to immobile form is recommended.
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