Estimation of Diffusion Coefficient of Chromium in Colloidal Silica Using Digital Photography

Abstract

In order to study the effectiveness of using colloidal silica, NYACOL DP5110, to stabilize chromium-contaminated soil, the diffusion of chromium in colloidal silica gel was estimated from laboratory experiments. To measure diffusion coefficients of chromium in the colloidal silica gel, a new measurement method based on digital photography was introduced. A series of experiments were designed and conducted to validate this new method and to estimate the diffusion coefficients of chromium in the colloidal silica gel. Accuracy of the proposed method was evaluated by several differentways. It was found that the apparent diffusion coefficient of chromium in colloidal silica gel ranged from 1.76 to 8.48 x 10(-10) m2/s depending mainly on the concentration of silica in the gel with chromium concentration less than 10(-2) M. Higher silica concentrations yielded lower 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; the partition coefficient was calculated to be 0.549 L/g. Mass balance calculations were performed to evaluate the accuracy of the proposed method and found that the measuring error was less than 6.5%. Based on the test data, the estimation of diffusion coefficients for chromium in colloidal silica gel using digital photography seems to be accurate and precise. This method is suitable for analyzing colored chemicals inside clear/white gels. From the results, it can be concluded that the gel behaves as a porous material with silica network forming continuous solid phase and its pore space saturated with water. The chromium ions diffuse in porous silica gel on a tortuous path. Therefore, the bulk diffusion dominates. Thus, the silica can be represented as a fix and impenetrable immersion in the solution. The presence of these motionless silica chains leads to an increase in the mean path of the diffusing molecules between two points in the system. On the basis of the test results, it can also be concluded that colloidal silica, NYACOL DP5110, for in-situ treatment of chromium-contaminated soils seems to be ineffective. Further research of more realistic simulation of diffusion and refined gel formulation with the capacity to convert the chromium to an immobile form is recommended.

Geochemical evidence for the application of nanoparticulate colloidal silica gel forin situcontainment of legacy nuclear wastes

“…24,39,41 The porewater in gelled colloidal silica also has an elevated pH, potentially enhancing retention of Sr and Cs through increasing surface complexation to (clay) minerals and the silica nanoparticles (20,21,(36)(37)(38). 24,25,38,39,41 However, a high ionic strength due to a saline accelerant used to induce gelling 5,42 could cause (increased) competition between cations in the accelerant with contaminants for surface complexation processes 41 inducing the release of Sr and Cs from mineral components into the colloidal silica porewater. This could lead to diffusion of contaminants through the gelled materials 42 and subsequent release of radionuclides into the groundwater.…”

“…24,25,38,39,41 However, a high ionic strength due to a saline accelerant used to induce gelling 5,42 could cause (increased) competition between cations in the accelerant with contaminants for surface complexation processes 41 inducing the release of Sr and Cs from mineral components into the colloidal silica porewater. This could lead to diffusion of contaminants through the gelled materials 42 and subsequent release of radionuclides into the groundwater. Thus, a comprehensive understanding of these competing impacts on the geochemistry, fate, mobility and speciation of contaminants, such as Sr and Cs, is required to underpin the use of nanoparticulate colloidal silica materials for contaminant control.…”

Remediation of chromite ore processing residue using solidification and stabilization process

“…, SG has 100% removal efficacy for removal of Cr(VI) present in water at pH 5.0. Tantemsapya and Meegoda conducted studies using colloidal silica to treat chromium contaminated soil and reported that chromium adsorbed to colloidal silica gel and diffusion coefficient of colloidal silica gel ranges from 1.76 to 8.48 x 10 −10 m 2 /s and is dependent on pH and silica concentration. However, in the present study, the reduction experiments were carried out with native COPR sample without altering the pH since it was not a feasible option, considering the quantum of COPR present in the selected site.…”

Mathematical modelling of contact dermatitis from nickel and chromium

“…const.––mol Ion diffusion coefficient1.32 Pan et al. (2002) and Tantemsapya and Meegoda (2004)Assumed the values for dead and live cells are the same…”