Peter Van Impe

DIFFUSION OF CALCIUM CHLORIDE IN A MODIFIED BENTONITE: IMPACT ON OSMOTIC EFFICIENCY AND HYDRAULIC CONDUCTIVITY

Chemically modified bentonites are being developed with the aim of preserving low hydraulic conductivity in the presence of potentially aggressive permeants in pollutant-containment applications. ‘Multiswellable’ bentonite (MSB) has been obtained by treating standard sodium bentonite with propylene carbonate. Research on the engineering properties of MSB has focused mainly on permeability and chemical compatibility. Solute diffusion and membrane behavior in MSB have not yet been investigated. A combined chemico-osmotic/diffusion test was performed on a MSB specimen using a 5 mM CaCl2 solution. Permeability with distilled water and with the 5 mM CaCl2 solution was measured prior to and after the chemico-osmotic/diffusion tests. The material exhibited time-dependent membrane behavior with a peak osmotic efficiency value (ω) of 0.172 that gradually shifted to zero upon breakthrough of calcium ions. Effective diffusion coefficients of calcium and chloride ions were in the range commonly described for untreated bentonite at similar porosities. After the chemico-osmotic/diffusion stage and permeation with 5 mM CaCl2, the hydraulic conductivity of MSB increased from 1.1 × 10−11 m/s to 7.0 × 10−11 m/s. The MSB was apparently converted into a calcium-exchanged bentonite at the end of the test. Prehydration and subsequent permeation might have contributed to elution of the organic additive from the clay. Further investigation is recommended to clarify the effect of prehydration on the hydraulic performance of MSB in the presence of potentially aggressive permeants.

Determination of consolidation parameters of dredging and industrial waste sludge.

The paper presents the laboratory setup and some preliminary results of large strain consolidation testing on several types of dredging and industrial waste sludge. The tests are performed in the framework of a larger research project – funded within the Flemish Environmental Technology Platform (MIP) - investigating the impact of additives, specifically coagulants and flocculants, on the consolidation behaviour of mineral sludges.. Large strain consolidation parameters are determined using an adapted version of the well-known seepage consolidation test setup (Imai 1979). Interpretation of the test results is based on an iterative calculation using large strain consolidation theory as presented by Abu-Heyleh (1992). This procedure should allow an optimisation of type, concentration and application method of additives for a specific material.